[cmucl-imp] Labor Day snapshot

[Raymond Toy]

The Sept snapshot has been tagged and binaries will be uploaded soon.

Notable changes in this snapshot:

- ASDF2 updated to version 2.017.
- Improve type propagation for LOAD-TIME-VALUE.
- Getting documentation of a structure via DOCUMENTATION no longer
  signals an error.
- Reduce unnecessary consing of SAPs in ROOM.
- Make stack overflow checking actually work on Mac OS X.  The
  implementation had the :stack-checking feature, but it didn't
  actually prevent stack overflows from crashing lisp.
- Fix rounding of numbers larger than a fixnum.  (This is an ancient
bug, going back to the original CMU sources, I think.)

Ray