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/wasm/src/code_translator.rs

@@ -834,6 +834,12 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
         Operator::F32Le | Operator::F64Le => {
             translate_fcmp(FloatCC::LessThanOrEqual, builder, state)
         }
+        Operator::RefNull => state.push1(builder.ins().null(environ.reference_type())),
+        Operator::RefIsNull => {
+            let arg = state.pop1();
+            let val = builder.ins().is_null(arg);
+            state.push1(val);
+        }
         Operator::Wake { .. }
         | Operator::I32Wait { .. }
         | Operator::I64Wait { .. }
@@ -902,9 +908,6 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
         | Operator::I64AtomicRmw32UCmpxchg { .. } => {
             wasm_unsupported!("proposed thread operator {:?}", op);
         }
-        Operator::RefNull | Operator::RefIsNull { .. } => {
-            wasm_unsupported!("proposed reference-type operator {:?}", op);
-        }
         Operator::MemoryInit { .. }
         | Operator::DataDrop { .. }
         | Operator::MemoryCopy