Fixed disclosure of local variables by `eval`; ref #3710

This patch fixes the latter part of #3710. We need to change
`struct REnv` to fix the former part of the issue.

mrbgems/mruby-eval/src/eval.c

@@ -205,9 +205,9 @@ create_proc_from_string(mrb_state *mrb, char *s, int len, mrb_value binding, con
  if (!e) e = c->ci[-1].env;
  e = (struct REnv*)mrb_obj_alloc(mrb, MRB_TT_ENV, (struct RClass*)e);
  e->cxt.c = c;
  e->cioff = c->ci - c->cibase - 1;
  e->cioff = c->ci - c->cibase;
  e->stack = c->ci->stackent;
  MRB_SET_ENV_STACK_LEN(e, c->ci[-1].proc->body.irep->nlocals);
  MRB_SET_ENV_STACK_LEN(e, c->ci->proc->body.irep->nlocals);
  c->ci->target_class = proc->target_class;
  c->ci->env = 0;
  proc->env = e;