Add a "Convention" object to allow different export/import scenarios (VA only or cross platform) about tonel-vast

[dalehenrich]

This is what I've done in Rowan for declaring conventions. Rowan supports Rowan, Monticello, and RowanHybrid conventions.

• The Rowan convention appears to be the same as your suggestion for VA-ENVY - the method and and class category fields are not overloaded with package name matching semantics. It would be easy enough for Rowan to support both names for this convention.
• The Monticello convention matches the one used by Pharo, Squeak, and GLASS
• RowanHybrid is a subset of the Monticello convention, where the class category must match the package name (case insensitive) and the method category must match the package name (leading *, case insensitive) ...

Declaring the convention at this point in the structure, means that all of the packages in a project need to use the same convention ...
Looking at the tonel-demos, I'm thinking that it would be practical to make the #convention a package property as well ...
In Rowan, I enforce the convention on read (throwing errors if the convention is not followed), Rowan enforces the convention in the image, so all written packages will follow the declared convention.