support linenoise in mirb
MSVC: fix C4146

C4146 (unary minus operator applied to unsigned type, result still
unsigned)
pool: use mrb_malloc_simple instead of mrb_malloc
Merge pull request #1829 from takkaw/mirb_linenoise

support linenoise in mirb
Merge pull request #1830 from cremno/pool-fixes

two pool fixes
Make type casts safer.
Add more warning detections
Add a size check. This code cannot be built on environments (UINT32_MAX > SIZE_MAX).