Check that variable lists are actually lists

Check that variable lists are actually lists

'let' and 'let*' document that their first argument has to be a list,
but don't check for that; instead, they allow (and silently ignore)
other types.  Introduce an explicit type check.

* src/eval.c (Flet, FletX): Check that the variable list is indeed a
list.
* test/src/eval-tests.el: Add unit tests.