Simplify io modules.

- nacl, osx both now just use posix for printing, which just
calls write(2, ...)

- windows doesn't declare wrapper stubs for _open, _close, etc.
These are handled by a pass in Target.cpp that adds the underscores
necessary on windows.

- bugfix in test_handle (wrong type for strlen)
Merge branch 'master' of github.com:halide/Halide