Make select and join combinators functions

[MajorBreakfast]

Currently the select and join combinators are methods. This leads to asymmetric code like this, which is harder to read (at least I always thought so):

future1.join(future2)

It has already been discussed in passing that making them symmetric by making them functions would be nice:

future::join3(future1, future2, future3)

This also makes them more similar to their already symmetric variadic macro counterparts join! and select! that work in async functions.