Test file for scanning a general text
Fix scanning of general text

Previously constructions such as |\toks0 | were considered as valid
general texts, because general texts were taken as the right-hand side
of an assignment to a toks register.  This is now fixed: a general text
is a filler, a begin-group token, a balanced text, and an explicit
end-group token.
Include a test for the last fix of general text
Move sections around, remove old material, rename an internal function
Rename more auxiliaries
Document primargs functions more precisely
Update test files for new auxiliary functions