PHP 8.1 support

[nikic]

The following PHP 8.1 RFCs are relevant:

• https://wiki.php.net/rfc/pure-intersection-types
• https://wiki.php.net/rfc/enumerations
• https://wiki.php.net/rfc/explicit_octal_notation
• https://wiki.php.net/rfc/noreturn_type
• https://wiki.php.net/rfc/final_class_const
• https://wiki.php.net/rfc/new_in_initializers
• https://wiki.php.net/rfc/first_class_callable_syntax
• https://wiki.php.net/rfc/readonly_properties_v2

[nikic]

Everything is implemented now, but I'm not really happy with the representation for first-class callables added in 13549aa.