Update to Rust master: String conversion updates

As of rust-lang/rust/29bc9c632eda71c6b4a8b35db637971953b58d03 / rust-lang/rust#18976 `FromStr, from_str` have been moved to `std::str`.
Merge pull request #45 from msiemens/patch-1

Update to Rust master: String conversion updates
Merge remote-tracking branch 'upstream/master'