It seems we weren't including stdint.h on Windows by mistake. Likely we
were getting it anywy from some other header, but it sounds like that
got changed in newer versions of MinGW and either way we shouldn't rely
on that.
Being that we include stdint.h on all platforms, I also changed os.h to
always use these types when defining the `int32` and friends sized
types. Ideally we get rid of those defines in the future but we can at
least use the stdint.h types going forward (as we already have).