Improve type definition for combinators (any, first, some)

[artemmolotov]

This allows to improve type definition. See #305.

You may also be interested in an alternative implementation (@psalm-* attributes only).

Examples:

$promises = [
    $promise1; // - Promise<ExampleClass>
    $promise2; // - Promise<string>
];

[$exceptions, $values] = yield \Amp\Promise\some($promises);

// $exceptions - Throwable[]
// $values     - array<array-key, ExampleClass|string>

$promises = [
    $promise1; // - Promise<int>
    $promise2; // - Promise<bool>
];

[$exceptions, $values] = yield \Amp\Promise\any($promises);

// $exceptions - Throwable[]
// $values     - array<array-key, int|bool>

$promises = [
    $promise1; // - Promise<ExampleClass>
    $promise2; // - Promise<string>
    $promise3; // - Promise<int>
];

$value = yield \Amp\Promise\first($promises);

// $value - ExampleClass|string|int

[kelunik]

Thanks!