Renamed local variables "from_collections_context".

Made them singular: "from_collection_contest" to match struct name.  These
were missed when the struct was renamed.
Renaming each_from_collections() function to singular.

Also made local variable from_collections_contest singular.  These were
missed when struct from_collection_context was renamed.
Changed sf_list_reversed() to reverse any kind of collection.

The function takes any collection type, not just an sf_list_t.

Removed unused function each_from_collection() and struct
from_collection_context.  Added static function build_reversed_list()
used by sf_list_reversed().

Added new unit test for sf_list_reversed() from an sf_array_t.