This bumps the minimum supported stable Rust version to 1.34.
The TryFrom and TryInto traits are still reexported to not break the
API, but these reexports are deprecated and will be removed in a future
release.
The previous version had a => required|optional|default token, this was
duplicating information for Option types and didn’t look very good.
This new version looks like a type, which can be either Required<_>,
Option<_> or Default<_>, and means the same thing.
