Fix evt chaperones for multiple-valued evts
Add contracts for synchronizable events
Adjust class/c to use the new strategy for the external
parts of a contract (field, method, init, absent clauses)

Still to do:

- delay the creation of the class that checks internal contracts
  until a new derived class is created from a contracted class.

- extend the new implementation so it handles interface contracts
  (currently the old version of the external contracts is still there,
  just to handle this case)

The first of those is what (I hope) will bring down the memory
footprint for class/c contracts.
unbreak racket: disable the new class/c in favor of the old one (for now)
Make module reader not return negative span when port location changes.
Make TR optimizer tests reset the port line counting.

This means new logging lines should not change the numbering of all existing
logs making diffs much cleaner.
Add delays to unboxed let to avoid optimizing twice.