Add tests for additionalProperties

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
KeywordDescriptorBuilder: rename .setSchemaEquivalence() to .setEquivalence()

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Comment in KeywordEquivalence about a possible alternative design

These equivalences guarantee that we don't rebuild keyword validators for
keywords/keyword combination that are equivalent. But then, the production of
this function, which is used in the equivalence for now, could also be used as
arguments to the keyword validators themselves.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>