[Feature] introduce an `of()` method

[phh]

Seeing this example:

$users = type($users)->as(Collection::class);

Really wants me to append an ->of(User::class) to make sure a Collection<User> is returned.

This could be also be used for arrays:
type($users)->asArray()->of(User::class)

[pascalbaljet]

I've been experimenting with a dedicated IterableType class. We could resolve it similar to not(), but that might be a bit verbose:

type($variable)->iterable()->asArray()->of(User::class);

Alternatively we could introduce a function that returns an IterableType instance:

iterable($variable)->asArray()->of(User::class);
iterable($variable)->as(Collection::class)->of(User::class);

[phh]

How would you suggest dealing with the key?

[pascalbaljet]

Maybe something like this:

iterable($variable)->as(Collection::class)->of(User::class, 'int');

// or...

iterable($variable)->as(Collection::class)->key('int')->value(User::class);