double "for" loop

Should we have a syntax
```
for x in X, y in Y list f(x,y)
```
(similar to python)
instead of the current
```
flatten for x in X list for y in Y list f(x,y)
```
which wastes a lot of time creating tons of nesting lists and then flattening the whole thing.
You can check with
```
time(for i in l list for j in l list {i,j};)
```
vs
```
time(flatten for i in l list for j in l list {i,j};)
```
that half the time is spent flattening.
I can try to implement this if there's some interest.
Somewhat related: #3971 (would help implement that too)

Start by reproducing the two timing examples in the issue and reading the related discussion in #3971. Then identify the parser and evaluator entry points for the existing nested `for` syntax; done means the proposed double-`for` form is agreed, implemented, and avoids the intermediate nesting and flattening cost.

Written by the indexing model from the issue text.