mempool.c: renamed from pool.c

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.

include/mruby/compile.h

@@ -8,6 +8,7 @@
 #define MRUBY_COMPILE_H
 
 #include "common.h"
+#include "mruby/mempool.h"
 
 /**
  * mruby Compiler
@@ -122,7 +123,7 @@ struct mrb_parser_heredoc_info {
 /* parser structure */
 struct mrb_parser_state {
   mrb_state *mrb;
-  struct mrb_pool *pool;
+  struct mrb_mempool *pool;
   mrb_ast_node *cells;
   const char *s, *send;
 #ifndef MRB_NO_STDIO

include/mruby/mempool.h

@@ -0,0 +1,13 @@
+/*
+** mempool.h - memory pool
+**
+** See Copyright Notice in mruby.h
+*/
+
+/* memory pool implementation */
+typedef struct mrb_mempool mrb_mempool;
+MRB_API struct mrb_mempool* mrb_mempool_open(mrb_state*);
+MRB_API void mrb_mempool_close(struct mrb_mempool*);
+MRB_API void* mrb_mempool_alloc(struct mrb_mempool*, size_t);
+MRB_API void* mrb_mempool_realloc(struct mrb_mempool*, void*, size_t oldlen, size_t newlen);
+MRB_API mrb_bool mrb_mempool_can_realloc(struct mrb_mempool*, void*, size_t);