Migrate to multi-crate layout. #92

Job	Run time
cargo check (msrv) (windows-latest)	1m 39s
cargo clippy (windows-latest)	2m 16s
cargo clippy (android) (armv7-linux-androideabi)	47s
cargo check (msrv) (macos-14)	51s
cargo check (msrv) (android) (armv7-linux-androideabi)	39s
cargo clippy (android) (aarch64-linux-android)	56s
cargo check (msrv) (ubuntu-latest)	48s
cargo check (msrv) (android) (aarch64-linux-android)	40s
cargo clippy (android) (x86_64-linux-android)	48s
cargo check (msrv) (android) (x86_64-linux-android)	45s
cargo clippy (macos-14)	59s
cargo clippy (ubuntu-latest)	58s
cargo doc (windows-latest)	1m 21s
cargo fmt	23s
cargo test (windows-latest)	1m 26s
cargo doc (macos-14)	42s
cargo test (macos-14)	45s
cargo doc (ubuntu-latest)	26s
cargo test (ubuntu-latest)	31s
	17m 40s

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