make upgrade/downgrade work for fusion sequences. New design, upgrade/downgrade aren't extension points anymore, but rather model_of is correctly defined to do 'the right thing', i.e. keeping the same register size. There might be compile-time overhead fixable by a smarter implementation or factorization with vector_of.
Merge pull request #805 from MetaScale/issue_802

make upgrade/downgrade work for fusion sequences. New design, upgrade/do...