Add comments in irep.h

Save mirb history when readline enabled

Fix string test for so that it works for int16

Changed use of MRB_INT_MAX to MRB_INT_MAX-1

Fix warning by making call explicit

Allow programmers to specify build path manually

Repalace int with mrb_bool because a return value is boolean.

Replace 0 with NULL because struct pointer is set NULL in.

Replace int with mrb_bool because a return value is boolean.

Replace 0 with NULL because set NULL in sturct pointer.

Replace 0 with FALSE and replace 1 with TRUE.

Add comments in variable.c

Replace mrb_intern() with mrb_intern2() in random.c

Replace mrb_intern() with mrb_intern2() or mrb_intern_cstr().

Replace 0 with NULL because NULL is used in struct pointer.

Add comments in opcode.h

Fix indents in opcode.h.