Mark l() as deprecated and add a function like rl() for route-based links

In Drupal 8, we should as much as possible remove reliance on Drupal system paths and instead work with routes and their parameters. To that end, l() shoudl be marked as deprecated, and we should add a new function that renders an A tag from a route name and parameters.

somewhat related issues:
#2046737: Add a method to the AccessManager that only needs a route name and parameters
#1979468: Stop adding the active class in l() and instead add it in JS