Auto merge of #53424 - RalfJung:miri-refactor, r=oli-obk

CTFE engine refactor

* Value gets renamed to `Operand`, so that now `interpret::{Place, Operand}` are the "dynamic" versions of `mir::{Place, Operand}`.
* `Operand` and `Place` share the data for their "stuff is in memory"-base in a new type, `MemPlace`. This also makes it possible to give some more precise types in other areas. Both `Operand` and `MemPlace` have methods available to project into fields (and other kinds of projections) without causing further allocations.
* The type for "a `Scalar` or a `ScalarPair`" is called `Value`, and again used to give some more precise types.
* All of these have versions with an attached layout, so that we can more often drag the layout along instead of recomputing it. This lets us get rid of `PlaceExtra::Downcast`. `MPlaceTy` and `PlaceTy` can only be constructed in place.rs, making sure the layout is handled properly. (The same should eventually be done for `ValTy` and `OpTy`.)
 This is used to check, when copying an operand to a place, that the sizes match (which caught a bunch of bugs).
* All the high-level functions to write typed memory take a `Place`, and live in `place.rs`. All the high-level typed functions to read typed memory take an `Operand`, and live in `operands.rs`.
* Remove `cur_frame` and handling of signedess from memory (catching a bug in the float casting code).
* [Only functional change] Enable sanity check to recurse below dyn traits and slices.

r? @oli-obk

Cc @eddyb