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
Hmph. Why was `bt` marked `@system` in `core.bitop`? That's rather unfortunate. But it's also out of the scope of this PR, so perhaps we should let that slide for now.