Performance tracking issue

[modulovalue]

Ready for PR:

• Render masks to constant lists. #23
• Specialize BitArray set ops to improve performance #18
• hashCode cache #22

Almost ready for PR:

• Specialize BitArray equality to improve performance. #21
• Consider using ctz for iteration. #19
• Investigate loop unrolling. #25

Needs more investigation:

• Consider using popcount for finding the cardinality of a BitArray. #20
• Investigate using Int32x4List as the underlying buffer for BitArray to make use of SIMD for faster set ops. #26

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Would benefit from efficient bit ops (dart-lang/sdk#52673):

• Consider using popcount for finding the cardinality of a BitArray. #20
• Consider making use of efficient implementations of popcount, ctz and clz if they become available. #17
• Consider using ctz for iteration. #19