Allow setting key's representation

[ip1981]

While working on a library to access TOML data via a "TOML path" like foo[1].bar, similar to toml-cli, we found convenient to reuse the Key struct. At the same time, reusing the key's parser provided by the public interface was not only tricky, but didn't cover some additional feature requirements (like wildcards). Another feature is that keys can come from a TOML file parsed with the toml_edit. That's why reusing Key is desired.

Current public interface does not allow creating a Key and setting its representation (Repr).

I propose making Repr::new_unchecked public and renamed to Repr::new (because Repr is self-contained, there are no invariants inside it, nothing to "check"), and making Key::with_repr_unchecked public as well, renamed to Key::with_repr.

These changes will allow writing custom parsers producing keys like this:

//...
.with_recognized()
    .map(|(o, i)| toml_edit::Key::new(o).with_repr(toml_edit::Repr::new(i)))
    .parse_next(input)

Here is the patch: https://github.com/whamcloud/toml/commit/7f9d96077b2cfd5fc30377304958be081c9e7b7f