This removes use of the pseudo-internal toString() function and also
removes extraneous uses of ReturnType<>. Also, a new when() method
is added to simplify additional uses.
The pattern we were using for actions required actions to be defined in
three places, an enum member containing the type string, an type
definition and a function. This combines all three of these into one by
using a helper function based on the ideas from [1].
[1]: https://phryneas.de/redux-typescript-no-discriminating-union
David Lechner