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.