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
After checking with the IRC, it seems like you could do this via a slice comparison
```D
if (p1[0..fullWords] != p2[0..fullWords])
    return 0;
```