Use `u8` in `Rounding`, and introduce `Roundingf` #13896

Job	Run time
cargo-deny aarch64-apple-darwin	38s
Check wasm32 + wasm-bindgen	2m 29s
Check wasm32+atomics	1m 33s
cargo-deny aarch64-linux-android	37s
Format + check + test	2m 17s
cargo-deny i686-pc-windows-gnu	39s
cargo-deny i686-pc-windows-msvc	37s
android	26s
cargo-deny i686-unknown-linux-gnu	36s
cargo-deny wasm32-unknown-unknown	37s
cargo-deny x86_64-apple-darwin	38s
ios	27s
cargo-deny x86_64-pc-windows-gnu	36s
cargo-deny x86_64-pc-windows-msvc	41s
cargo-deny x86_64-unknown-linux-gnu	39s
Run tests	2m 18s
cargo-deny x86_64-unknown-linux-musl	39s
cargo-deny x86_64-unknown-redox	41s
Check Windows	1m 40s
	18m 48s

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