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
@quickfur: Derp, yes, of course. Fixed it. There isn't a reason why it can't. The function is intended as a low-level intrinsic though, so...