chore: Rename `Limit` to `Head` #292

Job	Run time
Check documentation (Rust stable)	27s
Run tests (Rust stable)	24s
Check Formatting	20s
Run clippy (Rust nightly)	23s
Run tests with miri (Rust nightly)	1m 31s
Spell Check with Typos	6s
	3m 11s

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