Make anonymous traits only implementable in the same module that declared the type

[pcwalton]

What it says. Resolve is hard otherwise.

[brson]

Fine by me.

[catamorphism]

@pcwalton Is this still relevant? (do we even have anonymous traits?)

[catamorphism]

Closing -- I don't think we have anonymous traits.

[thestinger]

Maybe it means anonymous implementations?

[catamorphism]

@thestinger Maybe so; in that case, anon impls already have to be in the same crate that declared the type. Not sure whether tighting the restriction to "same module" improves anything?

[apoelstra]

#15060 is an example of a problem caused by the current behaviour.