Given that the `ZEND_AST_OP_ARRAY` type already needed special handling in
various places, it makes sense to give it its own struct to avoid some of the
casts. As a side benefit, it is a little smaller than the `zend_ast_zval`
struct.
For `zend_string` pointers in class entries, show the string contents (if not
`NULL`).
For class constants, the `ce` (class entry pointer) field is shown with the
name of the class, and the `doc_comment` field is shown with the string
contents if possible.
This inlines .gdbinit and php_gdb.py in the .debug_gdb_scripts section of the PHP binary so that GDB can auto-load them regardless of the current directory or the availability of the PHP source code (albeit some functionalities of php_gdb.py currently rely on the source being available).
