general: apply clippy lints (#407)

* general: update crates to 2021 edition.

* general: apply clippy --fix

* general: apply manual clippy lints

* cargo fmt.

Fix hidden lifetime parameter on `std::fmt::Formatter`

Add `#![warn(rust_2018_idioms)]` and `#![allow(dead_code)]` for 'webrtc-constraints'

Fix missing URL referenced in documentation comments

Fix inline URLs in documentation comments

Add missing documentation

Add macro-based `algorithms::select_settings::tests::smoke::macros` test

Clean up settings/constraints convenience macros, making them more consistent

Add bare-value constraint to `algorithms::select_settings::tests::smoke::…` tests

Make `algorithms::select_settings::tests::smoke::native` use unresolved constraints

Replace `flag.then_some(…)` with good old if-else to support MSRV 1.60.0

Improve test coverage for `MediaTrackConstraints`

Remove unused `MediaTrackConstraints::new(…)` & `MediaTrackConstraints::from(…)`

Improve test coverage for `BoolOrMediaTrackConstraints`

Remove unused `BoolOrResolvedMediaTrackConstraints`

Remove unused `ResolvedMediaStreamConstraints`

Improve test coverage of `MediaTrackConstraint`

Improve test coverage of `MediaTrackProperty`

Improve test coverage of `MediaTrackSetting`

Fix unit test for onverconstraining due to missing setting

Improve test coverage of `FacingMode` & `ResizeMode`

Improve test coverage of `MediaTrackCapability`

Remove unused constraint accessors

Add missing tests for `MediaTrackSupportedConstraints`

Add missing tests for `MediaTrackSettings`