Simplify loading of contribs
Describe the bug
Loading of contribs is inconsistent. For example, if asdf is loaded we can just
(require :sprof). But if we want the embedded-c contrib, we have to do
(require :cmu-contribs) and then
(require "embedded-c"). It would be much nicer if we we could just do
(require :cmu-embedded-c) to get that.
This means adding an asd file for each contrib. No current contrib uses defsystem, so we don't need to support that; just add asd file.
This would be especially useful if #79 (closed) were also done.