Refactor the value/neutral representation

Right now, value frames get "enriched" to include type information when they are applied to a neutral. There is a lot of thunking going on and this is making things kind of complicated. Ideally this wouldn't be needed at all.

I would like to move to something closer to https://github.com/AndrasKovacs/2ltt-impl/blob/main/src/Core/Value.hs. This change would touch a huge amount of code, which makes me think it is better to do it now than later.