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
minitest/minitest@ea9caaf
for `test-bundled-gems`.

lib/rdoc/context.rb

@@ -710,7 +710,7 @@ def display(method_attr) # :nodoc:
   # This method exists to make it easy to work with Context subclasses that
   # aren't part of RDoc.
 
-  def each_ancestor # :nodoc:
+  def each_ancestor(&_) # :nodoc:
   end
 
   ##