Mark _Word.allBits as @inlinable

[dnadoba]

Probably by accident @inlinable was left out here. Ideally we would eventually move to the official non underscore @inline(always) variant that implies @inlinable and would make this not look like it get's inlined but only within the module like it was before this patch. But that requires Swift 6.3+.

This has a surprisingly big effect on some algorithms. Over 10x for BitArray.fill(with: true) and BitSet.fromUnion given a Range:

Some other see "minor" improvements as well:

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[dnadoba]

This is the only real change, the rest is just benchmarks.

[lorentey]

D'oh, good catch! 💯