Add reference types R32 and R64

-Add resumable_trap, safepoint, isnull, and null instructions
-Add Stackmap struct and StackmapSink trait

Co-authored-by: Mir Ahmed <mirahmed753@gmail.com>
Co-authored-by: Dan Gohman <sunfish@mozilla.com>

cranelift/codegen/meta/src/shared/settings.rs

@@ -92,6 +92,18 @@ pub fn define() -> SettingGroup {
         true,
     );
 
+    settings.add_bool(
+        "enable_safepoints",
+        r#"
+            Enable safepoint instruction insertions.
+
+            This will allow the emit_stackmaps() function to insert the safepoint
+            instruction on top of calls and interrupt traps in order to display the
+            live reference values at that point in the program.
+            "#,
+        false,
+    );
+
     // Settings specific to the `baldrdash` calling convention.
 
     settings.add_enum(