Fiddle::Function responds to to_proc

This lets us cast a Fiddle::Function to a block, allowing is to write
things like:

```ruby
f = Fiddle::Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
define_method :strcpy, &f
```

ext/fiddle/lib/fiddle/function.rb

@@ -19,5 +19,10 @@ module Fiddle
     def to_i
       ptr.to_i
     end
+
+    # Turn this function in to a proc
+    def to_proc
+      lambda { |*args| self.call(*args) }
+    end
   end
 end

test/fiddle/test_function.rb

@@ -102,6 +102,19 @@ module Fiddle
       assert_equal("123", str.to_s)
     end
 
+    def call_proc(string_to_copy)
+      buff = +"000"
+      str = yield(buff, string_to_copy)
+      [buff, str]
+    end
+
+    def test_function_as_proc
+      f = Function.new(@libc['strcpy'], [TYPE_VOIDP, TYPE_VOIDP], TYPE_VOIDP)
+      buff, str = call_proc("123", &f)
+      assert_equal("123", buff)
+      assert_equal("123", str.to_s)
+    end
+
     def test_nogvl_poll
       # XXX hack to quiet down CI errors on EINTR from r64353
       # [ruby-core:88360] [Misc #14937]