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/src/regalloc/mod.rs

@@ -15,9 +15,11 @@ mod context;
 mod diversion;
 mod pressure;
 mod reload;
+mod safepoint;
 mod solver;
 mod spilling;
 
 pub use self::context::Context;
 pub use self::diversion::RegDiversions;
 pub use self::register_set::RegisterSet;
+pub use self::safepoint::emit_stackmaps;