perf: use get_unchecked for TwoWaySearcher

[KowalskiThomas]

What is this PR?

This is related to #27721.

This PR is a proposal for a performance improvement in std::pattern.

Profiling of https://github.com/quickwit-oss/quickwit in production shows that TwoWaySearcher::next is one of the most CPU-time-consuming functions, so I thought I would give it a look.
I read the contribution guide and this seems to be a fitting proposal.

It seems like TwoWaySearcher::next and TwoWaySearcher::next_back could be made faster by using get_unchecked in the inner loop comparisons instead of regular indexing, which is safe in the conditions where it would be done (indices are within bounds by construction).
I added some SAFETY comments in the code to explain why this is safe, as I believe is customary in those cases (and according to this page as well).

Benchmarks

I ran the existing bencharmks before/after the changes (only on my laptop, I can run them in other places if that's necessary).

./x.py bench library/coretests -- pattern::

We seem to be getting a ~7.5-12% performance improvement at a very low cost, which sounds worthwhile to me.
But this is the first time I'm proposing a change in Rust, so I'm looking forward to feedback on this.

BEFORE CHANGES
    pattern::ends_with_char   3398.91ns/iter +/- 526.28
    pattern::ends_with_str    3545.04ns/iter +/- 1108.76
    pattern::starts_with_char 3348.31ns/iter +/- 352.38
    pattern::starts_with_str  3710.59ns/iter +/- 435.57

AFTER CHANGES
    pattern::ends_with_char   3125.99ns/iter +/- 567.09  (-8.03%)
    pattern::ends_with_str    3106.43ns/iter +/- 258.33  (-12.38%)
    pattern::starts_with_char 3094.55ns/iter +/- 595.42  (-7.59%)
    pattern::starts_with_str  3365.75ns/iter +/- 268.88  (-9.29%)

System info for the benchmarks run

Details

Based on commit 8317fef20409adedaa7c385fa6e954867bf626fc

rustc 1.96.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: aarch64-apple-darwin
release: 1.96.0-dev
LLVM version: 22.1.2

Apple M4 Max
16
64 GB
ProductName:		macOS
ProductVersion:		26.3
BuildVersion:		25D125
(this was run on AC and without any heavy load from other apps or whatnot)

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