Use size_t for sizes/counts

[eschaton]

Rather than int, APIs that take or return sizes and counts should use size_t for consistency with C.

[brarcher]

As long as the change in the APIS wouldn't result in either build breakage from one of the platforms that Checks supports and won't introduce another kind of regressions, it sounds reasonable. I can't think what sort of practical regression there would be (int is signed and size_t is unsigned, for example). I'd be more curious to see if changing from int to size_t causes build warnings or error for some compilers.