util: Reimplement half <-> float conversions.

util: Reimplement half <-> float conversions.

Removed u_half.py used to generate the table for previous method.

Previous implementation of float to half conversion was faulty for
denormalised and NaNs and would require extra logic to fix,
thus making the speedup of using tables irrelevant.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>