FFI: Allow type retrieval of attached functions and global variables

This is the corresponding change to revealed type information introduced in
  ffi/ffi#1023

Author: larskanis

lib/truffle/truffle/ffi_backend/function.rb

@@ -137,6 +137,10 @@ def free
       @native_wrapper = nil
     end
 
+    private def type
+      @function_info
+    end
+
     private def convert_ruby_to_native(type, value, enums)
       if Primitive.is_a?(type, FFI::Type::Mapped)
         type.to_native(value, nil)
@@ -207,6 +211,8 @@ def free
   end
 
   class VariadicInvoker
+    attr_reader :return_type
+
     def initialize(function, args_types, return_type, options)
       @function = function
       @return_type = return_type

lib/truffle/truffle/ffi_backend/type.rb

@@ -112,6 +112,7 @@ def inspect
 
     class Mapped < Type
       attr_reader :native_type
+      attr_reader :converter
       alias_method :type, :native_type
 
       def initialize(converter)