Restructure building the C API (#7341)

This commit introduces a wrapper crate which is now the new "real" C
API. The purpose of this change is to enable using LTO when building the
C API. Currently

Restructure building the C API (#7341)

This commit introduces a wrapper crate which is now the new "real" C
API. The purpose of this change is to enable using LTO when building the
C API. Currently LTO is disabled because one of the crate types of the C
API is an "rlib" which means that it can't have LTO performed due to
rustc limitations. The solution here is to remove the "cdylib" and
"staticlib" crate types from the "wasmtime-c-api" crate, rename that
crate to "wasmtime-c-api-impl", and reintroduce 'wasmtime-c-api' as a
new crate which wraps the previous crate and reexports it.

This way LTO can be enabled when just building the artifacts and the use
case from #6765 is still satisfied by having a crate that can be linked
to from Rust. Locally this reduces the size of the C API artifact for me
by nearly 1M.

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