perf: remove loop from str::floor_char_boundary

[overlookmotel]

#144472 made str::floor_char_boundary a const function, but in doing so introduced a loop. This is unnecessary because the next UTF-8 character boundary can only be within a 4-byte range.

This produces excessive code for e.g. str.floor_char_boundary(20), because the loop is unrolled.

https://godbolt.org/z/5f3YsM6oK

This PR replaces the loop with 3 checks in series.

In addition to reducing code size in some cases, it also removes bounds checks from calling code when following floor_char_boundary with a call to String::truncate (which I assume might be a common pattern).

Notes

• I tried using index.unchecked_sub(1), but found it doesn't remove bounds checks, whereas index.checked_sub(1).unwrap_unchecked() does. Surprising!

• The assert_uncheckeds are required to elide checks from code following the floor_char_boundary call e.g.:

let index = string.floor_char_boundary(20);
string.truncate(index);

If this PR is accepted, I'll follow up with a similar PR for ceil_char_boundary.

Very happy to receive feedback and amend.

[rustbot]

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