Implement the LOOP Common Lisp macro

[pamoroso]

The LOOP macro is a major ANSI Common Lisp feature missing from Medley's implementation of Common Lisp, so it's worth adding. This issue is a place to coordinate the discussion and efforts to implement LOOP.

Some useful resources:

• Early implementations of Common Lisp's LOOP macro #1526
• LOOP implementation of the Sacla project

[masinter]

there was some fiddling with packages, but PR #1579 has the import of the Sacla implementation. I didn't add it to the loadup so you have to load CML-LOOP for now.

[masinter]

Modulo a bug or two (missing hash-table and package iterator #1584 ) this is completed.