[Fun] Add lazy() for doing lazy evaluations

[veewee]

This Fun\lazy() helper can be used for doing lazy evaluations:
https://en.wikipedia.org/wiki/Lazy_evaluation

Examples:

Lazy proxy:

$soap = Fun\lazy(fn () => new \SoapClient('http://slowwsdl.com'));
$functions = $soap()->__getFunctions();

Lazy evaluations:

$a = Fun\lazy(fn(): int => 1);
for ($i = 1; $i<10; $i++) {
    $b = $a;
    $a = Fun\lazy(fn() => $b() + $b());
}

$aResult = $a();
// = 2**10 -> all intermediate values are calculated lazily when calling the $a() callable.

Lazy streams

$incrementalNumbersStream = Fun\lazy(function() {
    $i = 0;
    while (true) {
        yield ++$i;
    }
});

take(10, $incrementalNumbersStream()); // [1...10]
take(20, $incrementalNumbersStream()); // [11...30]

[coveralls]

Pull Request Test Coverage Report for Build 1017034780

• 8 of 8 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0002%) to 99.935%

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Totals
Change from base Build 1017024431:	0.0002%
Covered Lines:	3072
Relevant Lines:	3074

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💛 - Coveralls

[azjezz]

Thanks @veewee 🎉