Function typecheck

typecheck<T extends string | ThingType>(
...types: T[],
): (thing: Thing<any>) => thing is OneTypeThing<T>
Return a helper function that returns true if the given Thing is any of the given types.
Type Parameters
- T extends string | ThingType
Parameters
- ...types: T[]
Returns (thing: Thing<any>) => thing is OneTypeThing<T>
Example
```
if (typecheck(ThingType.number, ThingType.string)(object)) {
    // inside this block, object is known to be
    // Thing<ThingType.number> | Thing<ThingType.string>
}
```