Make a gather/take construct

[masak]

I would be fine with having it be lexical rather than dynamic as in Perl 6. Lexical seems to rhyme better with what we can reasonably do with macros, and the dynamic part was never a dealbreaker anyway.

my values = gather {
    take 1;
    for [2, 3, 4] -> v {
        take v;
    }
}

Would desugar to something like

my _tmp_array;
_tmp_array.push(1);
for [2, 3, 4] -> v {
    _tmp_array.push(v);
}
my values = _tmp_array;

I also don't mind if the gather construct is not lazy, since we don't really have lazy lists/arrays in 007. The important thing is that everything works out nicely with a take finding its appropriate surrounding gather.

Note that there are bound to be some cases where a take is lexically inside the gather, but the `gather has completed already;

sub foo() {
    gather {
        take 1;
        return sub() { take 2 };
    }
}

foo()();    # ERROR: the `take 2` tried to take against a `gather` that has already finished