The union may have alignment requirements stricter than some of its
members. Some checking tools (e.g., gcc with options -fsanitize) can
then complain that the result of a cast from pointer to member to
pointer to the union is misaligned.

- Some api_checknelems changed to the more restrict api_checkpop.
- Added a class to the html for APIs in the manual.
- Comments and manual.

A close instruction is still inside the scope of the variables it is
closing. The extra close in a repeat-until (to close variables before
repeating the loop) was being coded outside that scope.