Implement a 'repeat while' macro

[masak]

The repeat while loop from Perl 6 comes in two flavors:

repeat {
    # ...
} while some_condition();

repeat while some_condition() {
    # ...
}

These both have the same semantics (evaluate the condition after each iteration, not before), but they also don't fall under #80, and so they would seem to require some sort of is parsed functionality.

[masak]

Hm. I wonder if the following use case doesn't indeed push my from being a statement form (like in Python) to being a term (like in Perl 6):

repeat while !valid_command(my command = prompt("> ")) {
    say("Please input a command: ");
}
# do something reasonable with `command`

I hesitate to complicate the language needlessly, but this arguably mostly moves complexity around. Arguably #199 wouldn't survive such a change, though. (Or would it? Could a multiple-variable my co-exist on the statement level with a single-variable my on the term level? At the very least, it'd introduce some degree of grammar ambiguity.)

[masak]

Coming back and reading the previous comment more than half a year later, and especially in the face of #279 (seemingly) taking us the other direction, I don't see how just declaring my command; on top of the loop is that arduous. The variable is meant to be used after the loop anyway, so why not just declare it visibly above the loop?

[masak]

What I really came in here to say is that we should make sure that repeat while works fine with next and last from #325. Which, yes, implies that we have some control flow protocol à la #145.