Fix Fiddle::Closure on TruffleRuby

* TruffleRuby requires a Proc or Method as the callable.

lib/fiddle/jruby.rb

@@ -185,15 +185,17 @@ def initialize(ret, args, abi = Function::DEFAULT)
       if ffi_args.size == 1 && ffi_args[0] == FFI::Type::Builtin::VOID
         ffi_args = []
       end
-      @function = FFI::Function.new(
-        Fiddle::JRuby.__ffi_type__(@ctype),
-        ffi_args,
-        self,
-        :convention => abi
-      )
+      return_type = Fiddle::JRuby.__ffi_type__(@ctype)
+      raise "#{self.class} must implement #call" unless respond_to?(:call)
+      callable = method(:call)
+      @function = FFI::Function.new(return_type, ffi_args, callable, convention: abi)
       @freed = false
     end
 
+    def to_ptr
+      @function
+    end
+
     def to_i
       @function.to_i
     end

test/fiddle/test_closure.rb

@@ -6,12 +6,6 @@
 
 module Fiddle
   class TestClosure < Fiddle::TestCase
-    def setup
-      if RUBY_ENGINE == "truffleruby"
-        omit("FFI::Function don't accept #call-able object with TruffleRuby")
-      end
-    end
-
     def teardown
       super
       # We can't use ObjectSpace with JRuby.
@@ -64,7 +58,7 @@ def call thing
     end
 
     def test_const_string
-      if RUBY_ENGINE == "jruby"
+      if RUBY_ENGINE == "jruby" or RUBY_ENGINE == "truffleruby"
         omit("Closure with :const_string works but " +
              "Function with :const_string doesn't work with JRuby")
       end
@@ -125,9 +119,14 @@ def test_memsize_ruby_dev_42480
       end
 
       require 'objspace'
+      closure_class = Class.new(Closure) do
+        def call
+          10
+        end
+      end
       n = 10000
       n.times do
-        Closure.create(:int, [:void]) do |closure|
+        closure_class.create(:int, [:void]) do |closure|
           ObjectSpace.memsize_of(closure)
         end
       end

test/fiddle/test_func.rb

@@ -64,10 +64,6 @@ def test_strtod
     end
 
     def test_qsort1
-      if RUBY_ENGINE == "truffleruby"
-        omit("TruffleRuby's FFI::Function don't accept #call-able object")
-      end
-
       closure_class = Class.new(Closure) do
         def call(x, y)
           Pointer.new(x)[0] <=> Pointer.new(y)[0]

test/fiddle/test_function.rb

@@ -94,10 +94,6 @@ def test_call
     end
 
     def test_argument_count
-      if RUBY_ENGINE == "truffleruby"
-        omit("TruffleRuby's FFI::Function don't accept #call-able object")
-      end
-
       closure_class = Class.new(Closure) do
         def call one
           10 + one