Renamed slist to forward_list following the C++ 2011 standard.

Open Watcom had a singly linked list container `_watcom::slist`. However the C++ 2011 standard calls for essentially the same thing as `std::forward_list`. This change renames the existing `_watcom::slist` template to `std::forward_list` (along with necessary updates to the build system and tests).

Existing programs making use of the old name will need to be revised to use the standard name. I believe there are very few such programs.