Add binding for Regexp#fixed_encoding?

It turned out we already had the method in C++, we just did not disclose
it to Ruby space.

lib/natalie/compiler/binding_gen.rb

@@ -1218,6 +1218,7 @@ def generate_name
 gen.binding('Regexp', 'casefold?', 'RegexpObject', 'casefold', argc: 0, pass_env: true, pass_block: false, return_type: :bool)
 gen.binding('Regexp', 'encoding', 'RegexpObject', 'encoding', argc: 0, pass_env: false, pass_block: false, return_type: :Object)
 gen.binding('Regexp', 'eql?', 'RegexpObject', 'eq', argc: 1, pass_env: true, pass_block: false, aliases: ['=='], return_type: :bool)
+gen.binding('Regexp', 'fixed_encoding?' ,'RegexpObject', 'is_fixed_encoding', argc: 0, pass_env: false, pass_block: false, return_type: :bool)
 gen.binding('Regexp', 'hash', 'RegexpObject', 'hash', argc: 0, pass_env: true, pass_block: false, return_type: :Object)
 gen.binding('Regexp', 'initialize', 'RegexpObject', 'initialize', argc: 1..2, pass_env: true, pass_block: false, return_type: :Object, visibility: :private)
 gen.binding('Regexp', 'inspect', 'RegexpObject', 'inspect', argc: 0, pass_env: true, pass_block: false, return_type: :Object)

spec/language/regexp/encoding_spec.rb

@@ -168,15 +168,11 @@
     make_regexp = -> str { /#{str}/ }
 
     r = make_regexp.call("été".dup.force_encoding(Encoding::UTF_8))
-    NATFIXME 'Implement Regexp#fixed_encoding?', exception: NoMethodError, message: "undefined method `fixed_encoding?' for an instance of Regexp" do
-      r.should.fixed_encoding?
-    end
+    r.should.fixed_encoding?
     r.encoding.should == Encoding::UTF_8
 
     r = make_regexp.call("abc".dup.force_encoding(Encoding::UTF_8))
-    NATFIXME 'Implement Regexp#fixed_encoding?', exception: NoMethodError, message: "undefined method `fixed_encoding?' for an instance of Regexp" do
-      r.should_not.fixed_encoding?
-    end
+    r.should_not.fixed_encoding?
     r.encoding.should == Encoding::US_ASCII
   end
 end