Generate C source file to represent mrb_irep structures.

Type `mrbc -S -B<init> -o<outfile> <rbfiles...>` to generate the C
source code that holds compiled `mrb_irep`.

Appending the following code to the bottom of the generated code,
`mruby` executes the compiled code:

```C
int
main()
{
  mrb_state *mrb = mrb_open();
  struct RProc *p = mrb_proc_new(mrb, &init_irep);
  mrb_vm_run(mrb, p, mrb_top_self(mrb), 0);
  mrb_close(mrb);
  return 0;
}
```

Eventually static compile should use this representation, instead
of `uint8_t` array that holds `mrb` data, so that we can skip
interpreting `mrb` data.

include/mruby/irep.h

@@ -65,6 +65,7 @@ typedef struct mrb_irep {
 
 #define MRB_ISEQ_NO_FREE 1
 #define MRB_IREP_NO_FREE 2
+#define MRB_IREP_STATIC  3
 
 MRB_API mrb_irep *mrb_add_irep(mrb_state *mrb);