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/filetests/src/lib.rs

@@ -46,6 +46,7 @@ mod test_postopt;
 mod test_preopt;
 mod test_print_cfg;
 mod test_regalloc;
+mod test_safepoint;
 mod test_shrink;
 mod test_simple_gvn;
 mod test_simple_preopt;
@@ -127,6 +128,7 @@ fn new_subtest(parsed: &TestCommand) -> subtest::SubtestResult<Box<dyn subtest::
         "simple-gvn" => test_simple_gvn::subtest(parsed),
         "verifier" => test_verifier::subtest(parsed),
         "preopt" => test_preopt::subtest(parsed),
+        "safepoint" => test_safepoint::subtest(parsed),
         _ => Err(format!("unknown test command '{}'", parsed.command)),
     }
 }