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

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

@@ -62,4 +62,10 @@ class Symbol
     self.length == 0
   end
 
+  def slice *args
+    to_s.slice(*args).to_sym
+  end
+
+  alias [] slice
+
 end

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

@@ -46,3 +46,13 @@ 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)
+end
+
+assert('Symbol#[]') do
+  assert_equal :a, :abc[0]
+  assert_equal :ab, :abc[0, 2]
+end