Support guards in maybe expression

[williamthome]

This PR implements guards in maybe expressions.
The motivation for the PR is this topic by @josevalim in Erlang Forums.

Example

-module(maybe_guard).
-feature(maybe_expr, enable).
-export([t/1]).

t(X) ->
    maybe 
        Zero when Zero =:= 0 ?= X,
	One when One > 0, One < 2 ?= one(),
	ok
    else
	X -> {nonzero, X}
    end.

one() -> 1.

1> maybe_guard:t(0).
ok
2> maybe_guard:t(1).
{nonzero,1}

Notes

There are things to fix, but this PR is to start the discussion for the implementation of this feature.

[github-actions]

CT Test Results

       3 files     374 suites   53m 30s ⏱️
2 639 tests 2 578 ✔️ 59 💤 2 ❌
7 028 runs  6 965 ✔️ 62 💤 1 ❌

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// Erlang/OTP Github Action Bot

[kikofernandez]

Sorry for the delay, we will look into this in the coming week or so.
Thanks!

[williamthome]

Hi, @kikofernandez.

No problem :)
I've not looked at this closely. This PR does not fully address the proposed.

Also, as mentioned in the Erlang Forum, this should be proposed in an EEP, and I agree to have things formalized. What do you or the OTP team think about this?

I can retake it or feel free to close this PR If the OTP team wants to assume this task.

[kikofernandez]

I am not sure yet. I like being able to add guards, but maybe it also makes sense if we think this could be applied to assignments, as also suggested in the Erlang Forums...

I will run it internally and see if there are some catches that I may not see.
I will keep you posted