improvements to `examples/ptb.dyna`

improvements to `examples/ptb.dyna`

 - (no longer a complete mess thanks to the new `load` commands)

 - does binarization

 - extracts and counts rules in a s-expression tree.

 - parses a sentence given as a string AND extracts the optimal parse by
   following backpointers.

sexpr loader uses nested lists as it's representation

'in'(+,+)+ asserts second arg is a list.

retract_rule: no longer crashes when retracting a backchained rule. There are
possible bugs because we haven't deleted the memo tables affected by that rule.

REPL: show changes after 'load' commands

String display format should no longer render special characters like '\n'