Move `Symbol#to_proc` to the core from `mruby-symbol-ext` gem.

Even though `Symbol#to_proc` is not included in ISO standard, the
`some_method(&:method_name)` is used very widely and convenient.
So we moved it to the core.

mrbgems/mruby-symbol-ext/mrblib/symbol.rb

@@ -3,12 +3,6 @@ class Symbol
 
   alias intern to_sym
 
-  def to_proc
-    ->(obj,*args,&block) do
-      obj.__send__(self, *args, &block)
-    end
-  end
-
   ##
   # call-seq:
   #   sym.capitalize  -> symbol

mrbgems/mruby-symbol-ext/test/symbol.rb

 ##
 # Symbol(Ext) Test
 
-assert('Symbol#to_proc') do
-  assert_equal 5, :abs.to_proc[-5]
-end
-
 assert('Symbol.all_symbols') do
   foo = [:__symbol_test_1, :__symbol_test_2, :__symbol_test_3].sort
   symbols = Symbol.all_symbols.select{|sym|sym.to_s.include? '__symbol_test'}.sort

mrblib/symbol.rb

+class Symbol
+  def to_proc
+    ->(obj,*args,&block) do
+      obj.__send__(self, *args, &block)
+    end
+  end
+end

test/t/symbol.rb

@@ -28,3 +28,7 @@ end
 assert('Symbol#to_sym', '15.2.11.3.4') do
   assert_equal :abc, :abc.to_sym
 end
+
+assert('Symbol#to_proc') do
+  assert_equal 5, :abs.to_proc[-5]
+end