buffer: floating point improvements

Improvements:
* floating point operations are approx 3x's faster
* NaN/Infinity assignation more closely follows d8's implementation
* all read/write on floating point now done in C, so no more need for
  lib/buffer_ieee754.js

I would have liked to do all assert checks in C, but unfortunately haven't been able to figure out how to properly check overflow when writing to a fast buffer.