Functions used in index definitions shouldn't be changed

Currently it is possible to change the behaviour of a function with
CREATE OR REPLACE FUNCTION even if the function is part of an index definition.

I think that should be forbidden, because it is very likely to corrupt
the index. I expect the objection that this would break valid use cases
where people know exactly what they are doing, but I believe that this
is a footgun for inexperienced users that should be disarmed.

I'd also opt for forbidding behaviour changing modifications with ALTER FUNCTION
for functions used in index definitions, specifically altering strictness.

Attached is a patch implementing a fix.

Yours,
Laurenz Albe