Add support for LOOP formatting to SEdit

[pamoroso]

Medley Common Lisp now supports loop but SEdit is understandably unaware of it, so it's unable to format and indent it properly. I tried making SEdit use the same formatting of IL:FOR for loop with (SEDIT:DEF-LIST-FORMAT CL:LOOP IL:FOR) but it doesn't make much difference.

It would be useful to eventually add to SEdit support for formatting loop.

[masinter]

I think another possibility although more far-reaching would be to not display a package identifier for those symbols that are not in the "right" package but are treated the same (name of command in the EXEC, local lexical variables, as well as CL:LOOP keywords that don't appear outside).

[hjellinek]

The SEdit docs are here: https://github.com/Interlisp/medley/tree/master/docs/internal/sedit