[plt-scheme] On PLaneT packages and compatible upgrades

To be honest, provide-version looks horribly wrong.

I would very much prefer a tool -- "Compatible Extension" -- that 
consumes two modules and either says "yes" or lists the things that one 
module exports and the other doesn't. In addition, it should come with 
an option "Generate Adapter," which produces a module m- with the same 
interface as m.v0 but with values from m.v1. (This should be N+2 lines: 
module, import plus one export per m.v0 interface.)

(1) I believe this tool is at the order of John's "simple checker" or 
whatever it's called.

(2) It is equally syntactic but it's a first cut.

-- Matthias