Rollup merge of #126787 - Strophox:get-bytes, r=RalfJung

Add direct accessors for memory addresses in `Machine` (for Miri)

The purpose of this PR is to enable direct (immutable) access to memory addresses in `Machine`, which will be needed for further extension of Miri.

This is done by adding (/completing missings pairs of) accessor functions, with the relevant signatures as follows:
```rust
/* rust/compiler/rustc_middle/src/mir/interpret/allocation.rs */

pub trait AllocBytes {
  // ..

  fn as_ptr(&self) -> *const u8;
/*fn as_mut_ptr(&mut self) -> *mut u8; -- Already in the compiler*/
}

impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes> {
  // ..

  pub fn get_bytes_unchecked_raw(&self) -> *const u8;
/*pub fn get_bytes_unchecked_raw_mut(&mut self) -> *mut u8; -- Already in the compiler*/
}
```
```rust
/* rust/compiler/rustc_const_eval/src/interpret/memory.rs */

impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
  // ..

  pub fn get_alloc_bytes_unchecked_raw(&self, id: AllocId) -> InterpResult<'tcx, *const u8>;
  pub fn get_alloc_bytes_unchecked_raw_mut(&mut self, id: AllocId) -> InterpResult<'tcx, *mut u8>;
}
```

r? ``@RalfJung``

Author: workingjubilee

src/alloc_bytes.rs

@@ -108,4 +108,8 @@ impl AllocBytes for MiriAllocBytes {
     fn as_mut_ptr(&mut self) -> *mut u8 {
         self.ptr
     }
+
+    fn as_ptr(&self) -> *const u8 {
+        self.ptr
+    }
 }