Implement 'next' and 'last'

[masak]

I'm not quite sure why we don't have them yet. I don't think they're all that difficult. We could probably emulate them with a control exception for now.

[masak]

And they probably need to be control flow exceptions anyway; see #145 (comment).

[masak]

Serious question: should (labeled) immediate blocks allow last? I find that surprising (an immediate block isn't a loop), but there's prior art: JavaScript does it that way.

What about other languages? Perl 5, Perl 6, Python, Java, C#, Ruby? I guess the landscape there can help determine what we do. If some of those languages don't allow breaking out of an immediate block, we might decide to follow suit. Using repeat while False (#160) to loop once seems more orthogonal anyway.

[masak]

• Perl 6 and Java definitely forbid last/break outside of any loop or switch. I was able to confirm this by trying.
• Perl 5 permits last in an immediate block. This, as far as I can tell, is because in Perl 5, an immediate block is a loop construct (so that the do ... while construct can work).
• Python doesn't have immediate blocks. (Or, just in case it does and I'm just not seeing them, it still only allows break from directly inside of loops.)
• I wasn't able to confirm either way with C# and Ruby.

...but on balance, I think we shouldn't do the last-from-immediate-block thing in 007. It's better to have simple, consistent rules.

[masak]

What I actually came into this issue to say is that I think we might as well do redo at the same time.

With redo too, there's the temptation to add it for not-loops. But (a) it actually changes the semantics, in case there's a nested block inside of the loop with a redo, and (b) it's more complicated, and sometimes simplicity is a virtue and generality is not.