No varargs

This removes all use of varargs from the halide_runtime, with the exception of calls to android and ios logging functions. This solves nacl linking issues and some windows mess (e.g. we couldn't print floats properly on windows).

Ready for comments.
CodeGen allocates a compile-time-dynamic size based on how large it could possibly get, with a saturation at 8192. I was too lazy to do this here. I could just change it to 8192, but that seems large.