Add String#inspect benchmark

(cherry picked from commit 893ebd4)

Author: eregon

bench/micro/string/inspect.rb

@@ -0,0 +1,19 @@
+# Copyright (c) 2020 Oracle and/or its affiliates. All rights reserved. This
+# code is released under a tri EPL/GPL/LGPL license. You can use it,
+# redistribute it and/or modify it under the terms of the:
+#
+# Eclipse Public License version 2.0, or
+# GNU General Public License version 2, or
+# GNU Lesser General Public License version 2.1.
+
+result = ""
+simple = "abcdefghij\nklmnopqrst\nuvwxyz"
+utf8 = "abçdéfghîĵ\nklmñöpqrst\nuvwxyz"
+
+benchmark "core-string-inspect-simple" do
+  result = simple.inspect
+end
+
+benchmark "core-string-inspect-utf8" do
+  result = utf8.inspect
+end