Seal the BitSet class

[jpountz]

Description

BitSet is used in some performance-critical places. We should seal this class so that it only allows FixedBitSet and SparseFixedBitSet as sub-classes, to prevent applications from mistakenly hurting performance by creating other sub-classes that could make call sites of BitSet#get, BitSet#nextSetBit and others polymorphic?

[priorigratia]

hi @jpountz can I work on this?

[jpountz]

Sure, feel free to give it a try. Be warned that this change may be controversial though, so it may not be merged even if the change is perfectly fine from the code perspective if the implications on users are deemed not good.