A Dictionary.removeValue(forKey:) benchmark regressed 35% because recent changes in this PR caused an _UnsafeBitMap member to not be inlined in its implementation. (This was probably triggered by moving a method from Dictionary._Variant to _NativeDictionary.)
Add @inline(__always) to _UnsafeBitMap members.
While we’re at it, make _UnsafeBitMap @usableFromInline. It’s only public for testing purposes.
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.
