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/frontend/src/ssa.rs

@@ -221,6 +221,8 @@ fn emit_zero(ty: Type, mut cur: FuncCursor) -> Value {
         cur.ins().f32const(Ieee32::with_bits(0))
     } else if ty == F64 {
         cur.ins().f64const(Ieee64::with_bits(0))
+    } else if ty.is_ref() {
+        cur.ins().null(ty)
     } else if ty.is_vector() {
         let scalar_ty = ty.lane_type();
         if scalar_ty.is_int() {