A LionWeb language is uniquely identified by its key + its version

[enikao]

In LionWeb, we consider two languages identical if they have the same key and the same version (by string comparison).
Behavior is undefined if two languages with the same key+version have different contents.

Ideally, the key SHOULD be chosen to be "globally" unique, but that's not a real (or enforceable) requirement.

Relates to #130