Remove the now-unused `VMGcRef::as_externref_unchecked` method (#9436)

Implement the Wasm GC instructions for converting between `anyref` and `externref` (#9435)

* Implement the Wasm GC instructions for converting between `anyref` and `externref`

This commit implement

Implement the Wasm GC instructions for converting between `anyref` and `externref` (#9435)

* Implement the Wasm GC instructions for converting between `anyref` and `externref`

This commit implements two instructions:

1. `any.convert_extern`
2. `extern.convert_any`

These instructions are used to convert between `anyref` and `externref`
values. The `any.convert_extern` instruction takes an `anyref` value and
converts it to an `externref` value. The `extern.convert_any` instruction takes
an `externref` value and converts it to an `anyref` value.

Rather than implementing wrapper objects -- for example an `struct
AnyOfExtern(ExternRef)` type that is a subtype of `AnyRef` -- we instead reuse
the same representation converted references as their unconverted reference. For
example, `(any.convert_extern my_externref)` is identical to the original
`my_externref` value. This means that we don't actually emit any clif
instructions to implement these Wasm instructions; they are no-ops!

Wasm code remains none-the-wiser because it cannot directly test for the
difference between, for example, a `my_anyref` and the result of
`(extern.convert_any my_anyref)` because they are in two different type
hierarchies, so any direct `ref.test` would be invalid. The Wasm code would have
to convert one into the other's type hierarchy, at which point it doesn't know
whether wrapping/unwrapping took place.

We did need some changes at the host API and host API implementation levels,
however:

* We needed to relax the requirement that a `wasmtime::AnyRef` only wraps a
`VMGcRef` that points to an object that is a subtype of `anyref` and similar for
`wasmtime::ExternRef`.

* We needed to make the `wasmtime::ExternRef::data[_mut]` methods return an
option of their associated host data, since any `externref` resulting from
`(extern.convert_any ...)` does not have any associated host data. (This change
would have been required either way, even if we used wrapper objects.)

* fix tests

* fix wasmtime-environ tests

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Add support for `#![no_std]` to the `wasmtime` crate (#8533)

* Always fall back to custom platform for Wasmtime

This commit updates Wasmtime's platform support to no longer require an
opt-in `RUSTF

Add support for `#![no_std]` to the `wasmtime` crate (#8533)

* Always fall back to custom platform for Wasmtime

This commit updates Wasmtime's platform support to no longer require an
opt-in `RUSTFLAGS` `--cfg` flag to be specified. With `no_std` becoming
officially supported this should provide a better onboarding experience
where the fallback custom platform is used. This will cause linker
errors if the symbols aren't implemented and searching/googling should
lead back to our docs/repo (eventually, hopefully).

* Change Wasmtime's TLS state to a single pointer

This commit updates the management of TLS to rely on just a single
pointer rather than a pair of a pointer and a `bool`. Additionally
management of the TLS state is pushed into platform-specific modules to
enable different means of managing it, namely the "custom" platform now
has a C function required to implement TLS state for Wasmtime.

* Delay conversion to `Instant` in atomic intrinsics

The `Duration` type is available in `no_std` but the `Instant` type is
not. The intention is to only support the `threads` proposal if `std` is
active but to assist with this split push the `Duration` further into
Wasmtime to avoid using a type that can't be mentioned in `no_std`.

* Gate more parts of Wasmtime on the `profiling` feature

Move `serde_json` to an optional dependency and gate the guest profiler
entirely on the `profiling` feature.

* Refactor conversion to `anyhow::Error` in `wasmtime-environ`

Have a dedicated trait for consuming `self` in addition to a
`Result`-friendly trait.

* Gate `gimli` in Wasmtime on `addr2line`

Cut down the dependency list if `addr2line` isn't enabled since then
the dependency is not used. While here additionally lift the version
requirement for `addr2line` up to the workspace level.

* Update `bindgen!` to have `no_std`-compatible output

Pull most types from Wasmtime's `__internal` module as the source of
truth.

* Use an `Option` for `gc_store` instead of `OnceCell`

No need for synchronization here when mutability is already available in
the necessary contexts.

* Enable embedder-defined host feature detection

* Add `#![no_std]` support to the `wasmtime` crate

This commit enables compiling the `runtime`, `gc`, and `component-model`
features of the `wasmtime` crate on targets that do not have `std`. This
tags the crate as `#![no_std]` and then updates everything internally to
import from `core` or `alloc` and adapt for the various idioms. This
ended up requiring some relatively extensive changes, but nothing too
too bad in the grand scheme of things.

* Require `std` for the perfmap profiling agent

prtest:full

* Fix build on wasm

* Fix windows build

* Remove unused import

* Fix Windows/Unix build without `std` feature

* Fix some doc links

* Remove unused import

* Fix build of wasi-common in isolation

* Fix no_std build on macos

* Re-fix build

* Fix standalone build of wasmtime-cli-flags

* Resolve a merge conflict

* Review comments

* Remove unused import

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Turn the `wasmtime-runtime` crate into the `wasmtime::runtime::vm` module (#8501)

* Expose `wasmtime-runtime` as `crate::runtime::vm` internally for the `wasmtime` crate

* Rewrite uses of `wasmtime

Turn the `wasmtime-runtime` crate into the `wasmtime::runtime::vm` module (#8501)

* Expose `wasmtime-runtime` as `crate::runtime::vm` internally for the `wasmtime` crate

* Rewrite uses of `wasmtime_runtime` to `crate::runtime::vm`

* Remove dep on `wasmtime-runtime` from `wasmtime-cli`

* Move the `wasmtime-runtime` crate into the `wasmtime::runtime::vm` module

* Update labeler for merged crates

* Fix `publish verify`

prtest:full

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