Add PlusPy's `Messaging.tla`

[lemmy]

PlusPy introduces a (generic) Messaging module that we should move to the CommunityModules.

/cc @johnyf

[johnyf]

Indeed, and if the Messaging module is removed from the repository of PlusPy, then some mechanism could be added there for fetching the CommunityModules (e.g., for running the test test8.scr, which uses the module TestBinBosco, which instantiates the module BinBosco, which instantiates the module Messaging).