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
The C/C++ programmer in me says yes, but in D I generally dislike implicit conversion to bool, 'cos in many cases it fails to convey programmer intent, which makes the code needlessly obscure.