[remove, please!] str::to_string() -> String's no longer found in the stdlib (as, at the very least, of of 29/5/14)

I've wandered into what I think's an appropriate direct substitute: String::from_str() seems eligible enough to typecheck mapping str to String, but I'd still suggest you give a probing one-over to these changes for idiomatic Rust style and semantics - I'm uncomfortable with most of the changes that've occurred -

- to_string calls on literals and results-of-slicing-operations have been turned into String::from_str calls, as noted earlier;

- another minor change, too; unrelated to the work here, but still String-related: a passage in mk_extern() passes references-to-Strings to to_intern_str instead of modified Strings (as required); I've decided to clone() them at the call site - I presume what is then passed in is allocated on the stack (and the compiler, I hope, will have finagled the escape analysis and have warned meif that's going to somehow involve memory dangled in the future.)