Hash functions correctly depend on _eq functions

[willmurnane]

When a function is marked with #[derive(PostgresHash, ...)] SQL code like the following is generated:

CREATE OPERATOR FAMILY Foo_hash_ops USING hash;
CREATE OPERATOR CLASS Foo_hash_ops DEFAULT FOR TYPE Foo USING hash FAMILY Foo_hash_ops AS
	OPERATOR    1   =  (Foo, Foo),
	FUNCTION    1   foo_hash(Foo);

However, in the pgx schema DAG, there is no dependency currently emitted between the OPERATOR CLASS and the foo_eq function (or the OPERATOR = associated with it). This means about half the time, the hash stuff is generated before the definition of equals, which means the extension will fail to install.

This merge request adds a dependency to the DAG, causing the equals function to always be emitted before the hash function. Closes #611 .

[Hoverbear]

Rest looks good!

[workingjubilee]

This looks lovely! Thank you!