Implement full callable type signatures

[nikita2206]

RFC https://wiki.php.net/rfc/callable-types

[faizanakram99]

Can this be revisited / reopened ?

[nikita2206]

@faizanakram99 looking at the PR probably doesn't make a lot of sense because a lot has changed in the PHP codebase since then. In general the feedback was that people didn't like the syntax of ad-hoc in-place type declarations, maybe this would fly with aliasing (ie if you had to declare function types like this type MyCallback = fn(Foo, Bar): int)