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/filetests/safepoint/call.clif

@@ -0,0 +1,52 @@
+test safepoint
+
+set enable_safepoints=true
+target x86_64
+
+function %direct() -> r64 {
+    fn0 = %none()
+    fn1 = %one() -> r64
+    fn2 = %two() -> i32, r64
+
+ebb0:
+    call fn0()
+    v1 = call fn1()
+    v2, v3 = call fn2()
+    brz v2, ebb1 
+    return v1
+ebb1:
+    v4 = call fn1()
+    return v3
+}
+
+; sameln: function %direct() -> r64 [%rax] fast {
+; nextln: ss0 = spill_slot 8
+; nextln: ss1 = spill_slot 8
+; nextln: sig0 = () fast
+; nextln: sig1 = () -> r64 [%rax] fast
+; nextln: sig2 = () -> i32 [%rax], r64 [%rdx] fast
+; nextln: fn0 = %none sig0
+; nextln: fn1 = %one sig1
+; nextln: fn2 = %two sig2
+; nextln: 
+; nextln: ebb0:
+; nextln:   v5 = func_addr.i64 fn0
+; nextln:   call_indirect sig0, v5()
+; nextln:   v6 = func_addr.i64 fn1
+; nextln:   v9 = call_indirect sig1, v6()
+; nextln:   v1 = spill v9
+; nextln:   v7 = func_addr.i64 fn2
+; nextln:   safepoint v1
+; nextln:   v2, v10 = call_indirect sig2, v7()
+; nextln:   v3 = spill v10
+; nextln:   brz v2, ebb1
+; nextln:   v11 = fill v1
+; nextln:   return v11
+; nextln: 
+; nextln: ebb1:
+; nextln:   v8 = func_addr.i64 fn1
+; nextln:   safepoint v3
+; nextln:   v4 = call_indirect sig1, v8()
+; nextln:   v12 = fill.r64 v3
+; nextln:   return v12
+; nextln: }