Optimize a lot and deprecate `to_u8` methods #46

Job	Run time
Lint	1m 20s
Test Rust source (stable, ubuntu-latest)	11s
Test Rust source (stable, ubuntu-latest, tracing)	9s
Test Rust source (stable, windows-latest)	42s
Test Rust source (stable, windows-latest, tracing)	1m 55s
Test Rust source (stable, macos-latest)	10s
Test Rust source (stable, macos-latest, tracing)	17s
Test Rust source (nightly, ubuntu-latest)	11s
Test Rust source (nightly, ubuntu-latest, tracing)	14s
Test Rust source (nightly, windows-latest)	22s
Test Rust source (nightly, windows-latest, tracing)	51s
Test Rust source (nightly, macos-latest)	11s
Test Rust source (nightly, macos-latest, tracing)	15s
Release crate	0s
Test crate release	0s
	6m 48s

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