Use bit arrays for predicate matching in search.

[isoos]

• Package search index improvements (tracking issue) #8671
• There are two further potential improvements, but I would implement them in a subsequent PRs: (a) reusing the bitarray instance, and (b) not creating/using a PackageScore instance when the BitArray would suffice.
• Measured around 5-10% improvement for predicate-only search, and around 1-4% for more complex text matching + predicates.

[isoos]

I've modified and benchmarked the 64-bit version of BitArray and couldn't point out much different performance to the current 32-bit implementation. With that in mind, I've vendored the library as-is, with minor updates (e.g. removing the part of and adding imports). I'll also update package:bit_array to reflect these changes, so next time the update will be straightforward file copy.

Should we also vendor some of the tests too?