perf!: Manual impl of `Region`/`FlatRegion` for improved perf #480

Job	Run time
miri	1m 20s
check	1m 9s
benches	1m 14s
tests (Rust 1.75)	1m 3s
tests (Rust stable)	46s
tests (Rust beta)	55s
tests (Rust nightly)	1m 10s
coverage	47s
rs-semver-checks / Rust semver-checks 🦀	1m 12s
	9m 36s

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