Closes GH-6709

Closes GH-6284.

Use the same names which are used by zend functions.

The "callable name" may be the same for multiple distinct callables.
The code already worked around this for the case of instance methods,
but there are other cases in which callable names clash, such as
the use of self:: reported in the referenced bug.

Rather than trying to generate a unique name for callables, compare
the content of the alfi structures. This is less efficient if there
are many autoload functions, but autoload *registration* does not
need to be particularly efficient.

As a side-effect, this no longer permits unregistering non-callables.

Replace EG(autoload_func) with a C level zend_autoload hook.
This avoids having to do one indirection through PHP function
calls. The need for EG(autoload_func) was a leftover from the
__autoload() implementation.

Additionally, drop special-casing of spl_autoload(), and instead
register it just like any other autoloading function. This fixes
bug #71236 as a side-effect.

Finally, change spl_autoload_functions() to always return an array.
The distinction between false and an empty array no longer makes
sense here.

Closes GH-5696.

This makes it always throw a TypeError, moreover this makes the
error message consistent.

Added a warning mentioning that the second parameter is now ignored
when passed false.

Closes GH-5301

Closes GH-5458

Not passing any parameters to this function has magic behavior.

Closes GH-5245