also updated the doc on `receiver_is_dispatchable` to reflect current state of the implementation

If object-safety checks succeed for a receiver type, make sure the
receiver’s abi is
a) a Scalar, when Self = ()
b) a ScalarPair, when Self = dyn Trait

Rename `CoerceSized` to `DispatchFromDyn`, and reverse the direction so that, for example, you write

```
impl<T: Unsize<U>, U> DispatchFromDyn<*const U> for *const T {}
```

instead of

```
impl<T: Unsize<U>, U> DispatchFromDyn<*const T> for *const U {}
```

this way the trait is really just a subset of `CoerceUnsized`.

The checks in object_safety.rs are updated for the new trait, and some documentation and method names in there are updated for the new trait name — e.g. `receiver_is_coercible` is now called `receiver_is_dispatchable`. Since the trait now works in the opposite direction, some code had to updated here for that too.

I did not update the error messages for invalid `CoerceSized` (now `DispatchFromDyn`) implementations, except to find/replace `CoerceSized` with `DispatchFromDyn`. Will ask for suggestions in the PR thread.

For a trait method to be considered object-safe, the receiver type must
satisfy certain properties: first, we need to be able to get the vtable
to so we can look up the method, and second, we need to convert the
receiver from the version where `Self=dyn Trait`, to the version where
`Self=T`, `T` being some unknown, `Sized` type that implements `Trait`.

To check that the receiver satisfies those properties, we use the
following query:

forall (U) {
if (Self: Unsize<U>) {
Receiver[Self => U]: CoerceSized<Receiver>
}
}

where `Receiver` is the receiver type of the method (e.g. `Rc<Self>`),
and `Receiver[Self => U]` is the receiver type where `Self = U`, e.g.
`Rc<U>`.

forall queries like this aren’t implemented in the trait system yet, so
for now we are using a bit of a hack — see the code for explanation.

Remove Ty prefix from Ty{Adt|Array|Slice|RawPtr|Ref|FnDef|FnPtr|Dynamic|Closure|Generator|GeneratorWitness|Never|Tuple|Projection|Anon|Infer|Error}

Co-authored-by: NTyler Mandry <tmandry@gmail.com>

Remove emulation of hygiene with gensyms

This is virtually certain to cause regressions, needs crater.

In #50781 it was discovered that our object safety rules are not sound because we allow `Self` in where clauses without restrain. This PR is a direct fix to the rules so that we disallow methods with unsound where clauses.

This currently uses hard error to measure impact, but we will want to downgrade it to a future compat error.

Fixes #50781.

r? @nikomatsakis

This commit moves the calculation of the `LanguageItems` structure into a
query rather than being calculated before the `TyCtxt` exists, with the eventual
end goal of removing some `CrateStore` methods.

Addresses the second part of #42171 by removing the `TraitRef` from
`ProjectionTy`, and directly storing its `Substs`.

Closes #42171.

Instead, thread around `Option<CodeExtent>` where applicable.