Replaced calls to external fmin/fmax by a Rust implementation.

Calls to the libc versions of fmin and fmax were relatively slow (perhaps because they could not be inlined?). This pull request provides f32 and f64 with fmin and fmax written in Rust, and shows a significant speed increase on my system; I used https://github.com/thiez/rustray as my benchmark, with --opt-level 3 it brings the ray-tracing time down from 10.8 seconds to about 9.2, which seemed significant to me.

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