Initial buffer_ieee754 Micro Optimizations

Attempting to bring writeIEEE754 performance closer to that of
DataView's setFloat32. These few small changes increase performance by
about 20%. They include:

- Removal of isNaN() checks. Arguments are controlled, so no need for
  that check.
- Variable rt wasn't caching a static result. Added that as constant at
  top of script.
- Inlined the absolute assignation of value.
- Assigned more values at beginning because of previous.