Examine whether the "return type" concept actually works, given overloads

[domenic]

Continued from https://www.w3.org/Bugs/Public/show_bug.cgi?id=25051 and #776.

The spec currently makes use of the concept of "return type", sometimes in reference to an operation, and sometimes in reference to a particular overload of an operation. An operation in general doesn't have a single return type, given the existence of overloads. Additionally, I think the spec also uses "return type" for things like callback functions.

#776 (comment) contains a list of all uses of "return type", which we should audit, figure out what they want to use, and perhaps come up with a better formal framework.

[annevk]

This also ties into whether a specification can describe an overloaded operation as a single operation. (And #674 is somewhat relevant too.)