size_t should be replaced by uint, not csize

[stefantalpalaru]

Since this tool is being used to interface with C and C++ libraries, we can't afford that whole "we'll change the C standard in our leaky abstraction" silliness, so what is an unsigned integer in C should be the same in the Nim wrapper.

Relevant Nim silliness: nim-lang/Nim#12187

[Araq]

Don't we have csize_t now in system.nim we can use instead?

[stefantalpalaru]

Yes, csize_t will be just as good, once it reaches a release.

[Araq]

Now uses csize_t.