In from expression, allow in to assign to pattern

[julianhyde]

Currently, the left-hand side of in must be a variable. We should allow it to be a pattern, to deconstruct rows coming from the right-hand side.

For example, we currently allow

from e in emps yield e.deptno + e.sal

but we should also allow

from {deptno = deptno, sal = sal, ...} in emps yield deptno + sal

and we currently allow

from p in pairs yield let val (left, right) = p in left + right end

but we should also allow

from (left, right) in pairs yield left + right

[julianhyde]

If the pattern contains constant elements then rows that do not match are skipped. For example,

= from (left, 2) in [("a", 2), ("b", 4), ("c", 2)]
-   yield left;
val it = ["a","c"] : string list

[julianhyde]

Fixed in b1f5955.