Create Primitives crate

[girazoki]

We need a primitives crate that hosts several of the traits that currently some pallets depend upon (e.g., getSessionIndex). We can also host some of the common types (ParaId, AuthorityId, etc) that we presumably are going to use in several runtimes (e.g., https://github.com/paritytech/cumulus/blob/master/primitives/core/src/lib.rs).