The type of each word used by IntSet is uint64,
but 64-bit arithmetic may be inefficient on a 32-bit platform.
Modify the program to use the uint type,
which is the most efficient unsigned integer type for the platform.
Instead of dividing by 64, define a constant holding the effective size of uint in bits, 32 or 64.
You can use the perhaps too-clever expression `32 << (^uint(0) >> 63) for this purpose.