Decouple framing and IntoSep

Currently, IntoSep does a few things that can be rather expensive, like AsFractional conversion. This is very useful for destruct patterns, but IntoSep is also used for framing, and there we want to be cheap. @tchajed already disabled some IntoSep instances in Perennial for this reason.

It might make sense to have a separate IntoSep for framing that is more optimized for performance and, for example, does not try to exploit fractional things.

This is somewhat similar to #186. To avoid making a mess, probably we should figure out #139 first.