extra semicolons removed from (some) .sig files

A complete re-implementation of lexing. Speeds things up considerably
through the use of mosmllex. (The example at the bottom of term_tokens.sml
goes through four to five times faster than it used to.) The basic
design is now more stateful than it used to be. There is a new type,
called the qbuf, which provides buffered access to a quotation. The
implementation of parse_type changes completely because it can no longer
do the monadic-parsing trick of backing up over failed parses. The
implementation of the term parser doesn't really change at all.