`ToStr` renamed to `ToString`

This fixes a [breaking change](https://github.com/rust-lang/rust/pull/15493) from `rust`. The ToStr trait, as well as its methods, were renamed.

Since the `ToString` trait was being used to implement `Show` for the `Message` type I renamed the method there as well.

Builds as of: `rustc 0.11.0 (f9fe251777e9f1cc557a6d5b82b45403935d0a10)`