mempool.h provide compatibility layer so that existing programs does not
need to update (but update recommended anyway, since compatibility layer
takes mrb_state that is not used at all).
To avoid confusion with pools in irep, we renamed region-based memory
manager from pool to mempool.
- rename pool.c to mempool.c
- separate mempool.h
- rename all mrb_pool to mrb_mempool
So if someone is using pool.c functions (I suppose no one does though),
they need to rename all `mrb_pool` to `mrb_mempool` and include
`mruby/mempool.h` header at the top.