implementation of bitsets

 This package implements bitsets, a mapping between non-negative integers
 and boolean values. It should be more efficient than map[uint] bool.
 .
 It provides methods for setting, clearing, flipping, and testing
 individual integers.
 .
 But it also provides set intersection, union, difference, complement,
 and symmetric operations, as well as tests to check whether any, all,
 or no bits are set, and querying a bitset's current length and number
 of positive bits.
 .
 BitSets are expanded to the size of the largest set bit; the memory
 allocation is approximately Max bits, where Max is the largest set
 bit. BitSets are never shrunk. On creation, a hint can be given for the
 number of bits that will be used.
 .
 Many of the methods, including Set, Clear, and Flip, return a BitSet
 pointer, which allows for chaining.