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/reader/src/lexer.rs

@@ -370,6 +370,8 @@ impl<'a> Lexer<'a> {
             "b16" => types::B16,
             "b32" => types::B32,
             "b64" => types::B64,
+            "r32" => types::R32,
+            "r64" => types::R64,
             _ => return None,
         };
         if is_vector {