buffer: floating point read/write improvements

Improvements:
* floating point operations are approx 4x's faster
* Now write quiet NaN's
* all read/write on floating point now done in C, so no more need for
  lib/buffer_ieee754.js
* float values have more accurate min/max value checks
* added additional benchmarks for buffers
* created benchmark/_bench_timer.js which is a simple library that
  can be included into any benchmark and provides an intelligent tracker
  for sync and async tests