Remove cowboy_dispatcher

Types and code are moved to cowboy_router. The match/3 export
from cowboy_dispatcher isn't available anymore as it is called
internally.
New routing

Ultimately few things change, it's mostly just a nicer syntax and
slightly different expectations. The name of the value `dispatch`
did not change, because the previous dispatch values will now fail
if the code is not updated to using `cowboy_router:compile/1`.

No constraints have been implemented in this commit.
Add the 'int' constraint
Add the 'function' constraint
If a binding is reused, we check that values are identical

This is more for consistency than anything.
Update roadmap
Update Ranch to 0.6.1