Fix warnings of clang -Wshorten-64-to-32 - #740

I think at least some (all?) of these warnings are actual problems, and rather than papering over them by casting to int, we need to take adjust the types used. e.g. printbuf_memappend() should take a size_t, int ret should be size_t ret instead, etc...

That might break API/ABI compatibility, so I'll need to take a closer look at each change.

I fixed some of these issues in commit bdd5e03. Changes to json_tokener.c and json_object.c are still needed.

Fixing this means at least changing the size and bpos members of struct printbuf to be size_t.
That is an ABI breaking change, and that doesn't even include changing the size of various function parameters too, so actually fixing this will need to wait until we're ready for a 1.0 release.

If you're going to change the API/ABI, please do use size_t instead of int for every object-size-related parameter in the API, and please use const on parameters taken by reference that are not going to be modified by the function call (i.e. "const struct json_object *jo" if *jo is not going to be modified by that API function).