Modify lexing of multi-char operators per pghackers discussion around

Modify lexing of multi-char operators per pghackers discussion around
16-Mar-00: trailing + or - is not part of the operator unless the operator
also contains characters not present in SQL92-defined operators.  This
solves the 'X=-Y' problem without unduly constraining users' choice of
operator names --- in particular, no existing Postgres operator names
become invalid.

Also, remove processing of // comments, as agreed in the same thread.