Write exotic float conversions in Rust.

[scott-griffiths]

The bitstring exotic floats go through an intermediate step for efficiency reasons. They truncate to 16 bits and can then use a lookup table.

This works OK, but it does mean that sometimes things can get rounded in the wrong direction. See scott-griffiths/bitstring#342

With bitformat we have a Rust core which should be pretty fast at this sort of thing, without needing the rounding approximation, so suggest it's implemented in Rust rather than porting over the LUT code.