Implement string comparsions in the JIT

[nbhuiyan]

Option 1: Replace std::string with internal string type (complex). This gives us a known layout for string, which would allow us to work directly with the underlying character array.
Option 2: Dispatch to a string comparison jit helper where operands are Values. (easy) Do a string looking up the helper, not in the generated code.

Will go with option 2 unless people have other ideas.