Prepare miri engine for enforcing validity invariant during execution
In particular, make recursive checking of references optional, and add a `const_mode` parameter that says whether `usize` is allowed to contain a pointer. Also refactor validation a bit to be type-driven at the "leafs" (primitive types), and separately validate scalar layout to catch `NonNull` violations (which it did not properly validate before).
Fixes https://github.com/rust-lang/rust/issues/53826
Also fixes https://github.com/rust-lang/rust/issues/54751
r? @oli-obk