deps: update v8 to 3.15.11
bindings: update to new v8 apis

GetPointerFromInternalField() is deprecated now, we should use
GetAlignedPointerFromInternalField().
test: make tests work with newer v8
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