Add String#intern and Hash#symbolize_keys DoS check

Currently CheckSymbolDoS looks for the unsafe usage of String#to_sym method. Except for this method, there is also synonymous String#intern method. Another possibility to turn user-supplied strings to new symbols is Hash#symbolize_keys method, which converts all keys to symbols as long as they respond to to_sym method.

This PR adds checks for String#intern, Hash#symbolize_keys and Hash#symbolize_keys! along with tests.