`dom` should not require a type annotation for the return type

For almost all uses, the return type of dom should be inferrable from the source type, so we should change the setup appropriately.

The only exception is taking, say, the dom coPset of a gmap — but that's only used once in Iris (or clients, according to @jung), and clients can convert the resulting gset to a coPset if they want.

I suspect we could change the modes for Dom:

-Global Hint Mode Dom ! ! : typeclass_instances.
+Global Hint Mode Dom ! + : typeclass_instances.

and then fix any overlapping instances.

(We could then change dom's return type to be implicit).

I'd start by removing the overlapping instances, and then if everything still builds we can change the mode and make the argument implicit.

but that's only used once in Iris (or clients, according to @jung),

FWIW I removed that usage with iris@bc1afae4

mentioned in merge request !376 (merged)

closed with merge request !376 (merged)

mentioned in commit 53c9d7f7