make `===` primary, deprecate `is`

[StefanKarpinski]

Since we now almost universally use the is function by the operator ===, we should make that the official name and make is a deprecated alias for === and eventually free up the name is.

[Ismael-VC]

and eventually free up the name is

@StefanKarpinski is there a new meaning for is in the horizon? Or do you just mean to free the name for user code?

[StefanKarpinski]

No new meaning – it just seems weird to have two names for the same operation for no reason.