Now `function_` has to return a valid value. `undefined` isn't a valid value

This lets you simplify the code from:

```js
const foo = await generate();
await cache.set('key', foo);
return foo;
```

to:

```js
return set('key', await generate());
```

Comments on https://stackoverflow.com/a/59126088/288906

This enables easy overloads support.

Disk caching helps functions that take a while to execute and sync functions generally don't.

If necessary, a user-side solution would be:

```ts
cache.function(async x => syncFunction(x))
```

There's no way to support overloads on sync functions: https://stackoverflow.com/q/59125705/288906

Types could be fixed to at least support overloaded async functions but I couldn't figure out how.