Introduce `LazySmallBitVector`

`LazySmallBitVector` is a bit set data structure which holds up to 31/63
bits in place (depending on the pointer size), and if more than that is
needed, the required space is automatically allocated on the heap.

`LazySmallBitVector` also features lazyness, meaning that any bit index
that has never been set will return `false`, while every time a certain
bit needs to be set the data structure is automatically enlarged, if
necessary.