Disallow reference to static mut for expressions

Add `E0796` error code.
Add `static_mut_ref` lint.

This is the idea for the 2024 edition.

compiler/rustc_error_codes/src/error_codes/E0796.md

@@ -0,0 +1,22 @@
+Reference of mutable static.
+
+Erroneous code example:
+
+```compile_fail,edition2024,E0796
+static mut X: i32 = 23;
+static mut Y: i32 = 24;
+
+unsafe {
+  let y = &X;
+  let ref x = X;
+  let (x, y) = (&X, &Y);
+  foo(&X);
+}
+
+fn foo<'a>(_x: &'a i32) {}
+```
+
+Mutable statics can be written to by multiple threads: aliasing violations or
+data races will cause undefined behavior.
+
+Reference of mutable static is a hard error from 2024 edition.