Improve allocation speed and reduce memory copying to increase HTTP pars...

...er performance by 1-2 (and occasionally more) percent in many cases such as benchmark/http/end-vs-write-end.js based on slab allocator todo by Ben.
Please wrap at 80 columns and please use single quotes.  (The latter is not a hard requirement but it's the norm in core and code that deviates always looks a little grating.)