Converts the parse tree PARSE-TREE into a REGEX object and returns it.

Will also
- split and optimize repetitions,
- accumulate strings or EVERYTHING objects into the special variable
STARTS-WITH,
- keep track of all registers seen in the special variable REG-NUM,
- keep track of all named registers seen in the special variable REG-NAMES
- keep track of the highest backreference seen in the special
variable MAX-BACK-REF,
- maintain and adher to the currently applicable modifiers in the special
variable FLAGS, and
- maybe even wash your car...