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=3568
- https://issues.dlang.org/show_bug.cgi?id=11015
I don't see how you can do it simply.
Also, any complex solution is out of scope for this already scope-creep'ed PR.