A bunch of changes to implement R7RS exit/emergency-exit.

A bunch of changes to implement R7RS exit/emergency-exit.

* The EXIT procedure has been removed.
* The %EXIT procedure has been renamed to EXIT.
* The EMERGENCY-EXIT procedure has been added.
* The optional argument to the above has been generalized to meet R7RS
  requirements.
* The QUIT procedure has been renamed to SUSPEND to more accurately reflect what
  it does.
* The names %EXIT and QUIT are deprecated aliases for EXIT and SUSPEND.