Use GATs in `System` and `ParSystem` associated types.

[Anders Evensen]

With rustc 1.65.0, GATs (Generic Associated Types) have been stabilized. That means we can now move the lifetime 'a from the System and ParSystem traits directly into the traits' Views associated types. This better indicates what exactly the lifetime 'a applies to (which is the lifetime of the view on the components).

This will, naturally, bump the MSRV to 1.65.0.