wasmtime: Add support for func.ref and table.grow with funcrefs

`funcref`s are implemented as `NonNull<VMCallerCheckedAnyfunc>`.

This should be more efficient than using a `VMExternRef` that points at a
`VMCallerCheckedAnyfunc` because it gets rid of an indirection, dynamic
allocation, and some reference counting.

Note that the null function reference is *NOT* a null pointer; it is a
`VMCallerCheckedAnyfunc` that has a null `func_ptr` member.

Part of #929

crates/environ/src/module_environ.rs

@@ -92,6 +92,10 @@ impl<'data> TargetEnvironment for ModuleEnvironment<'data> {
     fn target_config(&self) -> TargetFrontendConfig {
         self.result.target_config
     }
+
+    fn reference_type(&self, ty: cranelift_wasm::WasmType) -> ir::Type {
+        crate::reference_type(ty, self.pointer_type())
+    }
 }
 
 /// This trait is useful for `translate_module` because it tells how to translate