[RFC] [Draft] Add UUID extension

[kocsismate]

Native support for UUIDs is clearly missing from PHP, so the newly added UUID extension would fill this void. For now, only v7 is implemented, just for demonstration purposes.

I wanted to benchmark how a native implementation compares against symfony/uid and ramsey/uuid performance-wise. Here are the initial results (with the caveat that I have not verified yet the correctness of the native version, and monotonic ordering is not yet guaranteed):

PHP UUID - 50 iterations, 20 warmups, 10 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Median
PHP - UUID	0.07254	0.07463	0.00048	0.65%	0.07329	0.07316

Symfony UUID - 50 iterations, 20 warmups, 10 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Median
PHP - UUID	0.62739	0.63435	0.00137	0.22%	0.63016	0.62998

Ramsey UUID - 50 iterations, 20 warmups, 10 requests (sec)

PHP	Min	Max	Std dev	Rel std dev %	Mean	Median
PHP - UUID	1.04438	1.06381	0.00333	0.32%	1.04850	1.04795