Fix contains(String, String) by adding a method for that type

`contains(String,String)` is currently aways false, because it refers to `contains(itr,x) at reduce.jl:173`. Each element of the iterated string is a char, which is never `isequal` to a string.

This changes `contains(String,String)` to match the behavior most languages have, where it checks if one string contains a substring.