As proposed in SE-0107: UnsafeRawPointer:
Rename 'init(allocatingCapacity:)' to 'UnsafeMutablePointer.allocate(capacity:)'
Rename 'deallocateCapacity' to 'deallocate(capacity:)'
`allocate` should not be an initializer. It's primary function is to allocate
memory, not initialize a pointer.
The 'sizeInWords(forSizeInBits:)' function was supposed to return the
size in words, but returned the size in bits, overallocating the bitmap
by a factor of 32 or 64, depending on the platform.