Support for f32/f64.

[waywardmonkeys]

bindgen outputs f32/f64 instead of c_float/c_double now, so let's
support that here.

[gnzlbg]

Why does bindgen do that? If the platform does not define __STDC_IEC_559__ then f32 and f64 do not necessarily map to C's float and double. musl is an example of such a platform.

[gnzlbg]

That is, whether f32 and f64 are FFI safe is target-dependent.

[gnzlbg]

See: https://rust-lang.github.io/unsafe-code-guidelines/layout/scalars.html#fixed-width-floating-point-types

[gnzlbg]

cc @emilio: rust-bindgen should be generating libc::c_float and libc::c_double here. OTOH, it should not be generating libc::uint32_t and similar types for integers, but u32 instead. It is guaranteed that u32 matches C's uint32_t (https://rust-lang.github.io/unsafe-code-guidelines/layout/scalars.html#fixed-width-integer-types), and therefore the aliases should probably be deprecated from libc.

[emilio]

Yes, I think this is a bindgen bug. Should be trivial to patch, probably goes as far as bindgen goes, since even before my rewrite it was generating f32: rust-lang/rust-bindgen@cfdf15f#diff-9fd798837900e68cf440b740d0b7363bL2004