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/gen_inst.rs

@@ -650,6 +650,9 @@ fn typeset_to_string(ts: &TypeSet) -> String {
     if ts.specials.len() > 0 {
         result += &format!(", specials=[{}]", iterable_to_string(&ts.specials));
     }
+    if ts.refs.len() > 0 {
+        result += &format!(", refs={}", iterable_to_string(&ts.refs));
+    }
     result += ")";
     result
 }
@@ -677,6 +680,7 @@ pub fn gen_typesets_table(type_sets: &UniqueTable<TypeSet>, fmt: &mut Formatter)
                 gen_bitset(&ts.ints, "ints", 8, fmt);
                 gen_bitset(&ts.floats, "floats", 8, fmt);
                 gen_bitset(&ts.bools, "bools", 8, fmt);
+                gen_bitset(&ts.refs, "refs", 8, fmt);
             });
             fmt.line("},");
         }