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/settings.rs

@@ -390,6 +390,7 @@ mod tests {
              enable_nan_canonicalization = false\n\
              enable_simd = false\n\
              enable_atomics = true\n\
+             enable_safepoints = false\n\
              allones_funcaddrs = false\n\
              probestack_enabled = true\n\
              probestack_func_adjusts_sp = false\n\