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

tests/all/linker.rs

@@ -59,11 +59,11 @@ fn link_twice_bad() -> Result<()> {
 
     // tables
     let ty = TableType::new(ValType::FuncRef, Limits::new(1, None));
-    let table = Table::new(&store, ty, Val::ExternRef(None))?;
+    let table = Table::new(&store, ty, Val::FuncRef(None))?;
     linker.define("", "", table.clone())?;
     assert!(linker.define("", "", table.clone()).is_err());
     let ty = TableType::new(ValType::FuncRef, Limits::new(2, None));
-    let table = Table::new(&store, ty, Val::ExternRef(None))?;
+    let table = Table::new(&store, ty, Val::FuncRef(None))?;
     assert!(linker.define("", "", table.clone()).is_err());
     Ok(())
 }