Specialize map and map! for BitArrays
Specializing map is even more important for bitarrays than it is for generic
arrays since there can be a 64x speedup by working at the level of Int64
instead of looping bit-by-bit. There's no other way to perform operations at the chunk-level without either exporting more functions or mucking around with BitArray's internal fields.
This adds functor types that allow mapping across BitArrays 64-bits at a time. Performance of `map(&, x, y)` is now 300x faster and very competitive with `x & y`. In-place `map!` is even faster. The one downside here is that bitwise operations are not typically performed using comparisons... but otherwise a whole slew of new names would need to be exported (e.g., nor, nand, implies, etc.).
