util: add BitSet

Extracted from #30126.

This introduces the BitSet data structure, inspired by std::bitset, but with a few features that cannot be implemented on top without efficiency loss:

• Finding the first set bit (First)
• Finding the last set bit (Last)
• Iterating over all set bits (begin and end).

And a few other operators/member functions that help readability for #30126:

• operator- for set subtraction
• Overlaps() for testing whether intersection is non-empty
• IsSupersetOf() for testing (non-strict) supersetness
• IsSubsetOf() for testing (non-strict) subsetness
• Fill() to construct a set with all numbers from 0 to n-1, inclusive
• Singleton() to construct a set with one specific element.

Everything is tested through a simulation-based fuzz test that compares the behavior with normal std::bitset equivalent operations.