[cmucl-imp] Snapshot 2020-04

[Raymond Toy]

Snapshot 2020-04 has been tagged and binaries will be uploaded soon.

Minor changes for this snapshot:

* ASDF 3.3.4
* Tickets
  * #79: Autoload ASDF when calling require the first time.
  * #80: Use ASDF to load contribs.  No need to load cmu-contribs
    first.  Just `(require :contrib-demos)` instead of `(require
    "contrib-demos)`.
  * #81: Added contribs from Eric Marsden
  * #82: `bc` no longer required; uses `expr` when building the
    runtime.


--
Ray