Fix missing foreign count function.

[qxjit]

Also updated the documentation to more accurately reflect the function's
behavior.

I think the function could be given a better name to communicate its
behavior too. Since it's an exported function, I didn't rename it as
part of this fix.

[garyb]

Hm, yeah this is a slightly strange one. It's a specialised case of span really.

[qxjit]

Yeah, it's confusing enough that I actually thought dropWhile & takeWhile were doing the wrong thing until I located the original source for count.

[garyb]

Thanks for fixing it anyway!