smalloc: initial implementation

smalloc: initial implementation

smalloc is a simple utility for quickly allocating external memory onto
js objects. This will be used to centralize how memory is managed in
node, and will become the backer for Buffers. So in the future crypto's
SlabBuffer, stream's SlabAllocator will be removed.

Note on the js API: because no arguments are optional the order of
arguments have been placed to match their cc counterparts as closely as
possible.