This non-modifying function returns a new Bitset
object of the same maximum size as the original but which only contains
those values at the indices specified by the argument other.
Indices in other may be specified either as a vector of integers or as
another bitset. Please note that filtering by another bitset is not a
"bitwise and" intersection, and will have the same behavior as providing
an equivalent vector of integer indices.
filter_bitset(bitset, other)