Add fr_{lst, heap}_foreach() and tests thereof. (#4237)

Add fr_{lst, heap}_foreach() and tests thereof. (#4237)

Note that because we have to declare an fr_{lst, heap}_iter as well
as a variable of type pointer to the element type of the {lst, heap},
we must take the approach of fr_dlist_foreach_safe(). Correct uses
will thus look like they have unbalanced braces.