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
Perhaps consider using `fullWords` explicitly here and line 943? `n` isn't exactly a very descriptive name, and it's only used in 2 places.