Allow linker to detect shared definitions.

[xlauko]

Implement a special linker mode where it is able to claim that two declarations, each from a different module, are the same.

[pgoodman]

Ideally I'd like an API that gives some amount of visibility into linking, unification of linked entities, and also conflict resolution. Things like "rename by appending a .1" is a policy decision that should not be built-in and unchangeable, it should be part of some kind of API that is helping the linker do its job. Similarly, it's possibly the case that in linking two entities, we'll want to change the associated metadata (e.g. to choose the 'best' metadata), or that we'll want to use the associated metadata to say "these things actually are different."