Make a brk/sbrk emulation.

Make a brk/sbrk emulation.

Mac OS X seems to have some difficulties with brk/sbrk (maybe with the
4MB heap limit), and replace all the allocation logic will be prone to
errors, I'll add a new define and lib to emulate brk/sbrk using more
standard allocation methods. By default the heap is 64MB, it should be
enough.