Publicly expose either macro as for_both

[thomaseizinger]

See commit messages for details.

Resolves #57.

I've added d4f231d at the very end as a suggestion. Happy to drop it if it is undesired. My rationale was that a closure like syntax is

a) more inuitive
b) less error prone

If we allow the user to specify patterns, they might actually run into a situation where they produce a non-exhaustive match. If there is consensus, I can migrate the entire crate to use closure-like syntax but I wanted to get feedback on the idea first.

[cuviper]

Hmm. Note that Rust functions and closures also allow full patterns for their arguments, not just an identifier.

Also, a lot of the internal uses have ref or ref mut patterns, in part because we still claim support all the way back to Rust 1.12, whereas match ergonomics came in Rust 1.26. That's ancient history, so we probably don't need to support that, but patterns are still useful in general. IMO it would be fine to just document that your pattern has to be irrefutable for both sides, just as your expression has to end up valid both ways as well.

We could still give that closure syntax, if desired, or even let it support patterns both ways.

[thomaseizinger]

Friendly ping @bluss !

[cuviper]

I committed the syntax suggestion. I'm trying to get things together for a release...