Fix multiple Calc defmath errors (bug#46750)

Fix multiple Calc defmath errors (bug#46750)

Fix incorrect variable scoping in `let*`, `for` and `foreach`.
Fix loop variable value in `foreach` (should be element, not tail).
Fix function quoting, as in ('cons x y) -- didn't work at all.

Reported by Stephan Neuhaus.

* lisp/calc/calc-prog.el (math-define-exp, math-handle-foreach):
* test/lisp/calc/calc-tests.el: (var-g, test1, test2, test3, test4)
(test5, test6, test7, calc-defmath): Test various defmath forms.