Add --each-while and --map-when

A couple of more macros:

    hy> (--each-while [1 2 3 4 5] (< it 3) (print it))
    1
    2
    3
    hy>

```--each-while``` continues to evaluate the body form while the
predicate form is true for each element in the list.

```--map-when``` uses a predicate form to determine when to apply the
map form upon the element in the list:

    hy> (list (--map-when (even? it) (* it 3) [1 2 3 4]))
    [1, 6, 3, 12]