Retract of optimization in computeDenot

Needs a weakening of an assertion in assertErased
Fixed handling of ThisTypes in TypeErasure.

Need to be treated like TermRefs.
Better error message for adaptations of implicit methods

Ig an implicit method has a result type which does not match the expected
type, the new case fires. We avoid to diagnose missing arguments, because
that's really confusing for an implicit method.
Reinstantiated full tests.

The fix to Types in 9bb35512 (Retract of optimization in computeDenot)
allowed the two tests to compile "twice" again.