Avoid including standard headers when we don't need them

[hikari-no-yume]

I was motivated to do this by https://github.com/JuliaComputingOSS/llvm-cbe/issues/98, though alas, Clang will complain about incorrect definitions of standard library functions even if you don't include their headers. :)

In any case, I think this is better hygiene in general, IMHO.