Implement <<= and >>= for std.bitmanip.BitArray.

As [requested on the forum] (http://forum.dlang.org/thread/ofccpzlslcektihpipmv@forum.dlang.org).
I really-really hope this is inlined but experience tells me it's likely not.
I'd double check it - if not, then using empty template args `rollRight()(sizet_ ...)` would help.