Reorganize `mcall()` in `mruby-method`.

Use `mrb_exec_irep()`. If possible, re-entry into the VM will be suppressed.

Note that due to the effect of being a tail-call, the backtrace of `Method#call` will be lost, and it will look as if the target method was called directly.

This change fixes the problem of infinite loops when redefining methods that make block calls using `mruby-method`.

```console
% bin/mruby -e 'mm = method(:proc); define_method(:proc, ->(*a, &b) { mm.call(*a, &b) }); p proc { 1 }'
trace (most recent call last):
        [257] -e:1
        [256] -e:1:in proc
        [255] -e:1:in proc
        ...SNIP...
        [1] -e:1:in proc
-e:1:in proc: stack level too deep (SystemStackError)
```

1 files changed, 2 insertions, 0 deletions

diff --git a/src/vm.c b/src/vm.c
index d7f2f58d7..dbc8bd88a 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -585,7 +585,9 @@ mrb_exec_irep(mrb_state *mrb, mrb_value self, struct RProc *p, mrb_func_t postho
   else {
     mrb_value ret;
     if (MRB_PROC_CFUNC_P(p)) {
+      cipush(mrb, 0, CI_ACC_DIRECT, mrb_vm_ci_target_class(ci), p, ci->mid, ci->argc);
       ret = MRB_PROC_CFUNC(p)(mrb, self);
+      cipop(mrb);
     }
     else {
       int keep = (ci->argc < 0 ? 1 : ci->argc) + 2 /* receiver + block */;