Switch to stable TryInto/TryFrom traits

[Diggsey]

[djc]

The simple approach to this seems to run into coherence trouble:

error[E0119]: conflicting implementations of trait `std::convert::TryFrom<_>` for type `String<_>`:
   --> src/lib.rs:218:1
    |
218 | / impl<T> TryFrom<T> for String<T>
219 | | where
220 | |     T: AsRef<[u8]> + StableAsRef,
221 | | {
...   |
227 | |     }
228 | | }
    | |_^
    |
    = note: conflicting implementation in crate `core`:
            - impl<T, U> std::convert::TryFrom<U> for T
              where U: std::convert::Into<T>;