Merge pull request #2479 from take-cheeze/sym_slice

Implement `Symbol#slice` and `Symbol#[]` in mruby-symbol-ext.

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

@@ -69,4 +69,10 @@ class Symbol
     self.length == 0
   end
 
+  def slice *args
+    to_s.slice(*args)
+  end
+
+  alias [] slice
+
 end

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

@@ -53,3 +53,15 @@ end
 assert('Symbol#intern') do
   assert_equal :test, :test.intern
 end
+
+assert('Symbol#slice') do
+  assert_equal 'a', :abc.slice(0)
+  assert_equal 'ab', :abc.slice(0, 2)
+  assert_nil :abc.slice(4, 4)
+end
+
+assert('Symbol#[]') do
+  assert_equal 'a', :abc[0]
+  assert_equal 'ab', :abc[0, 2]
+  assert_nil :abc[4, 4]
+end