Turn debug-info back on.
Add examples of left_factor in action.  Added hack of impl of Ord for SymbolRegistry.
Don't run tests in parallel right now; they produce output that I'm
eyeballing.
Switch to TreeMap to get deterministic output from tests.

Still puzzling over output from left rec elimination, I think the
rterm in this looks broken:

   <rterm> ::= <rterm> <rfactor>
   <rterm> ::= <rfactor>
 <rfactor> ::= <rfactor> "*"
 <rfactor> ::= <rprimary>
<rprimary> ::= "a"
<rprimary> ::= "b"

==>

     ...
     <rterm> ::= "*" <rfactor_2> <rterm_4>
     ...