Cleanup type declarations to be symmetric with other function definitions.

[eile]

6bdb3f9 is in #206. Will rebase once merged.

[eile]

Yes, we had discussed:

• the function typedefs for subscriber did not follow the same pattern
• I feel that one file, one type should be a rule with the exceptions of typedefs in types.h

Imo the only alternative would be making this sub-types of the class, which is done in other places.

[eile]

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