Rust has changed to_str to to_string

Update to_str to to_string to prevent the compiler
from erroring due to missing symbols etc.

I've also changed lit_to_str to lit_to_string to match, but I haven't
changed lit_is_str to match.  I'm not sure what is right, I'm just
trying to use the module.  When I know more I guess I'll have an
opinion.
Merge pull request #2 from pcn/change-to-str-to-string

Rust has changed to_str to to_string