general: apply clippy lints (#407)

* general: update crates to 2021 edition.

* general: apply clippy --fix

* general: apply manual clippy lints

* cargo fmt.

Fix inline URLs in documentation comments

Add missing documentation

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

Add unit tests for `fn fitness_distance(…)`, specifically individual type-combinations

Change order of items in match statement

Wrap bare property names into new-type `MediaTrackProperty` for type-safety

Replace exact with fuzzy floating-point comparisons (using normalized epsilon)

Rename `MediaTrackConstraint` and related types, adding `Resolved…` prefix

Remove `SettingFitnessDistanceErrorKind::Invalid` error variant

Refactor constraint tests for recent changes made to `SettingFitnessDistanceError`

cons

Extract logic of step 7 of the `fitness distance` algorithm into `fn relative_fitness_distance(…)`

(https://www.w3.org/TR/mediacapture-streams/#dfn-fitness-distance)

Make `ValueConstraint<T>`, `ValueRangeConstraint <T>`, `ValueSequenceConstraint<T>` & `MediaTrackConstraint` return new `SettingFitnessDistanceError`

cons

Rename `SettingFitnessDistanceError` to `SettingFitnessDistanceErrorKind` and re-introduce the former as a struct

cons

Add tests for distance function implementations of `ValueSequenceConstraint<T>`

Add tests for distance function implementations of `ValueRangeConstraint<T>`

Add tests for distance function implementations of `ValueConstraint<T>`

Add implementation of distance function for calculating setting fitness