Add Cmd::Yield for complex custom bindings

[tailhook]

This partially replaces #293, i.e. some custom bindings can be implemented like this, although using Yield-style bindings is more complex so PR's are mostly unrelated.

The idea behind the Yield building is that we return from the readline function with a special error value which holds user's value (Arc<dyn Any>) along with current input. This may be used by user's REPL loop for many complex to implement but useful things like:

1. Changing prompt
2. Changing mode of operation
3. Changing Vi / Emacs mode like I've put in the yield_binding example as part of this PR

What do you think?

[rwestphal]

What's the status of this PR?

I'm not a maintainer but these changes look good to me.

@tailhook maybe you could move the matches!-related changes and the new example into separate commits to facilitate the review process.

[tailhook]

@tailhook maybe you could move the matches!-related changes and the new example into separate commits to facilitate the review process.

I can move them before, not after. The issue is that Cmd doesn't have PartialEq after this change, so you can only match on it.

But generally I've not got any agreement that this functionality would be accepted. So I don't want to waste time on rebasing it until that. (And this PR is terribly outdated)

Also, the biggest issue with Yield being used for complex commands is that "undo" would not work well (this is still fine for other use cases, where you treat input differently if submitted by a different key binding, or if you have an external editor on a keybinding, though).

[rwestphal]

But generally I've not got any agreement that this functionality would be accepted. So I don't want to waste time on rebasing it until that. (And this PR is terribly outdated)

Fair enough. In the meanwhile I'll create a private fork of rustyline since I really need Cmd::Yield to implement a Cisco IOS-like CLI in Rust.

I hope this PR gets the attention it deserves as having the ability to bind custom behavior to certain keystrokes is crucial for any readline implementation.

[rwestphal]

LGTM

[Keating950]

Is this still being considered? I'm very interested in this feature.