[k2] support covariance for future<T>

[apolyakov]

This PR addresses an issue related to type handling in futures. Consider the following example:

$f1 = fork(...); // future<int>
$f2 = fork(...); // future<array<int>>

$arr = [$f1, $f2]; // []future<mixed>
wait($f1);
wait($f2);

In this example, the array $arr is expected to contain elements of type future<mixed>. However, at the time of creation, the futures have different types: future<int> and future<array<int>>. To ensure correct type handling and conversion, the compiler needs to insert runtime type conversions.

This PR enables the generation of such runtime type conversions.

[PetrShumilov]

LGTM