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

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

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

Fix unit test for onverconstraining due to missing setting

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

Change order of items in match statement

Make unit tests for overconstrained `fn apply_mandatory_constraints` check for proper adherence to provided exposure modes

Add basic unit tests for tie-breaking policies

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

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

Add fallback for `total_cmp` for Rust versions earlier than 1.62

Change Rust edition from 2021 to 2018, backfilling explicit imports of `std::iter::FromIterator`

Rename `BareOr…` types, removing prefix

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

Split implementation of `SelectSettings` algorithm into separate sub-modules for easier testing

Add basic tests for `fn select_settings(…)`

Move tie-breaking policies into separate sub-module

Extract `fn select_settings_candidates(…)` variant without tie-breaking from `fn select_settings(…)`

Make functions accept `IntoIterator` rather than `Iterator`, where appropriate

Rename `SelectSettingsPolicy` to `TieBReakingPolicy`

Make `fn select_settings(…)` accept `IntoIterator` rather than `Iterator`