show warning for unused block

With verbopse mode (-w), the interpreter shows a warning if
a block is passed to a method which does not use the given block.

Warning on:

* the invoked method is written in C
* the invoked method is not `initialize`
* not invoked with `super`
* the first time on the call-site with the invoked method
  (`obj.foo{}` will be warned once if `foo` is same method)

[Feature #15554]

`Primitive.attr! :use_block` is introduced to declare that primitive
functions (written in C) will use passed block.

For minitest, test needs some tweak, so use
ea9caafc07
for `test-bundled-gems`.

pack.rb

@@ -17,6 +17,7 @@ class String
   #  returns that array.
   #  See {Packed Data}[rdoc-ref:packed_data.rdoc].
   def unpack(fmt, offset: 0)
+    Primitive.attr! :use_block
     Primitive.pack_unpack(fmt, offset)
   end