This changes GMP functions to accept a GMP|string|int union with
standard semantics (and thus also uses it in function signatures).
Relative to the previous behavior, this means that GMP functions
in weak mode now also accept float and null, and in strict mode no
longer accept bool, and have full type information.
Closes GH-6139.
* get_parent_class() argument is optional
* Mark array_filter() $callback as optional
* The $base of gmp_strval() is optional
* DateTime constructor also accepts zero arguments
* hash_update_file() stream context is optional
* xmlwriter_write_dtd_entity() $isparam argument is optional
