Improve performance when working with arrays

[Athari]

Implementations of some methods can be replaced with a single call to built-in functions if underlying iterator is wrapping an array, for example toValues, toKeys, max etc. Use built-in functions whenever possible.

[Athari]

After some testing, this feature turned out to be largely impossible without making users of the library use numerous predefined static lambdas, which becomes highly inconvenient in the light of planned removal of string lambdas. Where '$k' was possible, now Functions::$key would be required, instead of typical for the modern PHP fn($v, $k) => $k. So even if that feature is implemented, it'll be obscure for the vast majority of users and a tough sell for those who are aware.

As such, this feature is abandoned.