Loosen mmap offset signature for BitArray types

[samuelpowell]

Documentation states that the signature for mmap to BitArray types is identical to that of normal arrays, though in fact the type required for the offset is more specific.

(This is pertinent because these methods can be put to good use in manipulating memory mapped registers on embedded systems, where it is natural to express an offset in, e.g., /dev/mem/ using the word size of the machine, often < 64 bits.)

[mbauman]

Mind adding a simple test for this case?

[samuelpowell]

@mbauman sure - do you need an explicit @test statement or will cd2e8f6, which will error in the case of a regression, suffice?