Fix erasure of trait info

After erasure, traits always extend object, and no other class.

The change flushed out three more problems, one in the handling of
Super trees in erasure, another in bridge method generation. and a
third that class RepeatedParam had Seq, which is a trait, as first parent.
Simplified code a bit
Fixed formatting (wrong indentation)
Merge pull request #264 from dotty-staging/fix/trait-erasure

Fix erasure of trait info