Delegate the care of a directly given block from cipop() to cipush()

Outlines:
  - Removed `mrb_callinfo::blk`
  - Added `mrb_callinfo::flags`
  - Added `MRB_CI_COMPANION_BLOCK` flag

include/mruby.h

@@ -176,9 +176,9 @@ typedef struct {
   uint8_t n:4;                  /* (15=*) c=n|nk<<4 */
   uint8_t nk:4;                 /* (15=*) */
   uint8_t cci;                  /* called from C function */
+  uint8_t flags;                /* MRB_CI_COMPANION_BLOCK or zero */
   mrb_sym mid;
   const struct RProc *proc;
-  struct RProc *blk;
   mrb_value *stack;
   const mrb_code *pc;           /* current address on iseq of this proc */
   union {