std.bitmanip BitArray refactor

I reviewed BitArray and found a few issues:
- constness
- incorrect treatment of "pad" bits
- many places where size_t should have been used.

Incidental fixes:
- https://issues.dlang.org/show_bug.cgi?id=11015
I didn't change this behaviour, I'm just documenting that it may behave different than you'd expect.
Once a solution to prevent stomping is implemented the warning can be removed.