buffer: implement new fill behavior

Old fill would take the char code of the first character and wrap around
the int to fit in the 127 range. Now fill will duplicate whatever string
is given through the entirety of the buffer.

Note: There is one bug around ending on a partial fill of any character
outside the ASCII range.
src: add node_isolate's from stable merge

Add node_isolate argument to new code from recent v0.10 branch merge.
smalloc: implement new v8 api

Implements some of the new v8 API that'll be coming. Hopefully this will
help instantiation time in the long run.
buffer: add yank() method

When a buffer comes in from the SlabAllocator or another pool it's
useful to quickly create a clone of that buffer in an unpooled instance
if known the slice will hang around for a while.