Clean up dump.c

dump_irep_record(): Set error code in case dump failed.

add context to parser, that would hold local variable info, filename, and line number.  mrbc_context argument has been added to mrb_parse_xxx() functions.  Normally, you just to need to add NULL (or 0) to the last argument of the above functions.

modify compiler API; replace mrb_compie_xxx with mrb_load_xxx() that combines compilatoin and execution

Initialize mt of module with khash.

Fix str_replace in string.c

Free iv of MRB_TT_DATA instance.