Wrap Link.__call__

Currently users directly override Link.__call__. I propose letting users override, say, Link.call instead, and call it from Link.__call__. It’s just like how Function.__call__ works.

This will introduce several benefits (At least I can think of):

• Link hooks
• Links will be able to be represented hierarchically in graph visualization. (See #2215)

Some candidate method names: call, __call_link__, define, define_link, any other?

Note: This will not break backward compatibility, as existing code (overriding Link.__call__) will just work, but with new features (e.g. link hooks) disabled.

[mitmul]

If there’s no objection to the above comment of @niboshi, I’m going to start writing this feature with the first option in the comment and send a PR soon.

[niboshi]

@mitmul Got it!