This PR:

• moves us to more consistent usage of the apollo.blah() way of sending extension methods through a namespace. This requires some silliness with Protocols With Associated Types for a couple things involving generics, but I think the consistency is worth it.
• Adds some more readable extension methods on URLs. I plan to update the Swift Codegen setup with this once it's merged, since it makes things a LOT clearer.
• Moves a couple things, including Atomic, to ApolloCore. This is mostly going to serve as a check that migrating more things to core isn't going to break everything for now, but I plan to move more stuff not necessary outside of reading in code-generated files into Core to help deal with Generated types import the entire Apollo runtime #1047