OS-specific I/O streams

[paulfryzel]

Currently there are symbol lookup errors on OS X when running tests via make test.

run-time error: run-time error: dlsym(RTLD_DEFAULT, stdout): symbol not found

We should remove the OS-specific declarations from the stdio bindings (as the libc headers differ, e.g. FILE *stdout (linux) -vs- FILE *__stdoutp (darwin)) and instead build them from their POSIX-compliant file descriptor identifiers.