This PR adds support for PHP enumerations.

In general, the :php:enum: and :php:case: directives work just like :php:class: and :php:const:, respectively. However, to support backed enums, I got a little creative and introduced a minor new syntax to the signature line.

Here's a very simple, basic enum example:

.. php:enum:: Suit
    .. php:case:: Hearts
    .. php:case:: Diamonds
    .. php:case:: Clubs
    .. php:case:: Spades

Here's the same example as a backed enum:

.. php:enum:: Suit : string
    .. php:case:: Hearts : 'H'
    .. php:case:: Diamonds : 'D'
    .. php:case:: Clubs : 'C'
    .. php:case:: Spades : 'S'

This new syntax is effectively the same as this:

.. php:enum:: Suit() -> string
    .. php:case:: Hearts() -> 'H'

The use of () ->, however, is intended to convey method parameters and a return type, which isn't appropriate semantics for a backed enum type and case values, so I decided to introduce this new syntax.

When it renders, Sphinx displays it with the arrow symbol (→) as if it is a return type, but we don't have much choice, since we must use one of the methods from sphinx.addnodes, and I chose to use desc_returns().