[syntax] "for" statement with multiple containers

[momentarylapse]

It is mildly inconvenient needing to write nested loops. It gets worse when using for to unwrap optionals and having to deal with failure states:

for a in aaa
    for b in bbb
        ....
    else
        error handling...
else
    same error handling...

It would be nicer to merge for statements

for a, b in aaa, bbb
    ...
else
    ...