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/wast/src/wast.rs

@@ -23,6 +23,7 @@ fn runtime_value(store: &Store, v: &wast::Expression<'_>) -> Result<Val> {
         F64Const(x) => Val::F64(x.bits),
         V128Const(x) => Val::V128(u128::from_le_bytes(x.to_le_bytes())),
         RefNull(RefType::Extern) => Val::ExternRef(None),
+        RefNull(RefType::Func) => Val::FuncRef(None),
         RefExtern(x) => Val::ExternRef(Some(ExternRef::new(store, *x))),
         other => bail!("couldn't convert {:?} to a runtime value", other),
     })
@@ -420,6 +421,7 @@ fn val_matches(actual: &Val, expected: &wast::AssertExpression) -> Result<bool>
                 false
             }
         }
+        (Val::FuncRef(x), wast::AssertExpression::RefNull(wast::RefType::Func)) => x.is_none(),
         _ => bail!(
             "don't know how to compare {:?} and {:?} yet",
             actual,