Move array_caches[] to on s_arena.
Move s_cache defined in main_code.c to on s_arena.
Remove saved_arena. (cont.)
Add "foreign import jhc_context ccall" for pass gc and arena to RTS.
Call jhc_alloc_init and jhc_hs_init at exported function's enter.
Remove saved_gc and saved_arena.
Call jhc_alloc_fini at exported function's enter.
The new_arena function takes a new arena from free_arena list.
Pass rtstest.
Merge pull request #22 from master-q/feature/reentrant2

Feature/reentrant2: Remove saved_gc and saved_arena