Use existential trick to define pattern synonym instead?

[edsko]

I'm not sure if it's possible, but perhaps we can use the existential types trick to instead define a pattern synonym without generating all accessors (without relying on NoFieldSelectors). If this is possible, this might avoid some copying at runtime; currently updating a field implies copying the record to a vector, then updating the vector, then copying back.

This might be in slight tension with #66 .