Enable frozen string across project (#1319)

Turn on frozen string support in all files by using a comment to avoid
enabling across dependencies where it may not work.

Fixes: #1177

spec/support/binding_proc.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 ##
 # A simple extension of the Proc class that supports setting a custom binding
 # and evaluates everything in the Proc using the new binding.