Flipping the direction of WithVal.equiv

Stream: mathlib4

[Kenny Lau]

I've heard it said that while simp lemmas are of the form [complex] = [simple], isomorphisms should actually go the other way round, because you use it to construct terms in the more complicated type.

With this in mind, I suppose we should flip the direction of WithVal.equiv?

(This would be a very small change, it's only used 8 times (Flipping WithLp.equiv would be a different topic tho...))

For more context, this came up in #30133.