HB.instance should check for duplicate mixins

[gares]

if the same mixin can be built in two different ways, both from the factories provided or from the existing instances, they must be convertible.

[gares]

Refactor code of declare-instances as follows:

• given T find all mixins: findall (mixin-for T M X)
• check duplicate results are just aliase, i.e. for a given M, all the Xs should be convertible
• then, find all the classes that can be inhabited using these mixins, by exhausting the mixins rather than the classes

Also, refactor get-canonical-mixins-of to get only one mixin-src per mixin, (because we ourselves maintain the invariant of convertibility). Maybe add a HB.check_instance_convertibility for debugging?

EDIT by Cyril

[CohenCyril]

In fact, it appears that the important check to do is when doing HB.instance Definition _ := F.Build T ..., the mixins generated by F that were already declared for T are compatible. Otherwise fail with a nice error message, e.g. The factory F tries to build `m1` ... `mn` in a way which is incompatible with previous instances on `T`, please use a "smaller factory" (and even better if we can suggest factories)