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
You mean `@trusted`?
OK, so `bt` will actually index an array of `size_t`? Is there any reason it can't take an array as argument to begin with?