This PR moves the `PredTrans.apply` structure field into a separate
`def`. Doing so improves kernel reduction speed because the kernel is
less likely to unfold definitions compared to structure field
projections. This causes minor shifts in `simp` normal forms.
This PR makes `mvcgen` reduce through `let`s, so that it progresses over
`(have t := 42; fun _ => foo t) 23` by reduction to `have t := 42; foo
t` and then introducing `t`.
