- Make API more idiomatic (x holdUnless y, instead of holdUnless(x, y))
- Add new SeqMem API, readAndHold, which corresponds to most common
use of holdUnless
This makes it possible for the bundles to have different widths.
Previously, we had to widen all the bundles passing through a node
to the widest of all the possibilities. This would mean that if
you had two source[] fields, they end up the same.