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.

Fixes compilation with:
$ rustc --version
rustc 0.11.0-nightly (1c711db551b9a5e56ba76e002e287dcfbb2cff3c 2014-07-09 00:36:40 +0000)