Kill off complications in CoreFVs
When doing type-in-type, Richard introduce some substantial complications in CoreFVs, gathering types and free variables of type. In Trac #13160 we decided that this complication was unnecessary, so this patch removes it. Unfortnately I then fell down a twisty rabbit hole. Roughly: * An apparently-innocuous change in the AnnApp case of fiExpr made the fuction a little bit stricter, so we ended up peering into the arguments when we didn't before (namely when there are no bindings being floated inwards). I've rejigged it so that it's not fragile any more. * Peering into the arguments was sometimes expensive, becuase exprIsExpandable can be expensive because it looks deeply into the expression. * The combination of the two led to a non-linear explosion of work when the argument of a function is a deeep nest of constructors. This bug has been lurking for ages. I solved it by replacing exprIsExpandable with exprIsHNF + exprIsTrivial; see Note [noFloatInto considerations] * The code around floating case-expressions turned out to be very delicate, because can_fail primops (which we want to float inwards) can't be floated outwards; so we have to be careful never to float them too far. Note [Floating primops] has the details * I ended up refactoring some rather opaque code in sepBindsByDropPoint. Working all this out meant that I rewrote quite a bit of code, so it's now a reasonably substantial patch. But it's a net improvement.