3.4.16 `last-for?' - detect last iteration

Usage: (last-for? [ for_var ])
Returns SCM_BOOL_T if the named FOR loop (or, if not named, the current innermost loop) is on the last pass through the data. Outside of any FOR loop, it returns SCM_UNDEFINED. See section 3.6.13 FOR - Emit a template block multiple times.

Arguments:
for_var - Optional - which for loop