Change Traject.__call__ to Traject.consume.
Use composition instead of inheritance for Traject.
Eliminate fallback based on priority, instead issue TrajectError
as conflicts.
No more converter weight needed.
use generalized for the discriminator, meaning types don't matter
anymore.
No more need for discriminator converters.
use new-style converters for path variable decode and parameter encode.
Rewrite to use explicit converter infrastructure.
No more NameParser, as we pass converters separately from path now.
Remove more hardcoded converter logic.
Give App a context. Context variables are ignored when processing
URL parameters.
Refactor mounting story.

Instead of special hidden context variable, the Mount class has
become a WSGI app. The app WSGI app can only be used if the app
has no variables, otherwise .mounted must be called first.
Get rid of the rather mixed up notion of a 'base'.

The whole get_base and base functionality was only minimally tested and
was complicating the code. Get rid of it. Also rename 'base' to 'parent'
where it was in use inside mounts.