Support joining Promises of different types about promise

[danielt1263]

The above is how it's done in, for example, RxSwift. They have a file with functions that allow up to 8 parameters.

[khanlou]

I like your code gen function!

In this library, what you call join is called zip. Currently, it only allows two parameters, but we could add higher arity versions.

The changes that I'm thinking about making to zip are a) removing the and parameter name, and b) moving to a new enum namespace named Promises, so that you dont have to specify a generic type that isnt used anywhere

[juri]

How did I manage to miss that? zip seems kind of obvious in retrospect, but I managed to scroll right past it when browsing the source. Oh well. I'll rework the code gen to work with it.

[juri]

This was partially invalid and the rest was implemented in #15. Closing.