Fixed visibility at method definition

There was a problem with visibility state from proc that straddles a fiber or is independent.

Therefore, it has been changed to give priority to env objects, if any.
Also, added "separate module" flag to block traversal to a higher level env object.

Note that the "separate module" flag is now set when calling blocks with the `mrb_yield_with_class()` function.

fixed https://github.com/mruby/mruby/issues/6494

include/mruby.h

@@ -176,7 +176,8 @@ 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 vis;                  /* visibility in the current scope */
+  uint8_t vis;                  /* 5(ZERO):1(separate module):2(method visibility) */
+                                /* under 3-bit flags are copied to env, and after that, env takes precedence */
   mrb_sym mid;
   const struct RProc *proc;
   struct RProc *blk;