Add first-class callables

Support acquiring a Closure to a callable using the syntax
func(...), $obj->method(...), etc. This is essentially a
shortcut for Closure::fromCallable().

Add first-class callables

Support acquiring a Closure to a callable using the syntax
func(...), $obj->method(...), etc. This is essentially a
shortcut for Closure::fromCallable().

RFC: https://wiki.php.net/rfc/first_class_callable_syntax

Closes GH-7019.

Co-Authored-By: Nikita Popov <nikita.ppv@gmail.com>

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