Prevent pitfalls in route configuration

[CathalMullan]

Think of edge-cases which we can't/shouldn't support.
e.g.

• touching dynamic parameters /{1}{2}
• two routes ending in a dynamic: /{1}, /{2}
• wildcard next to wildcard: /{*1}/{*2}
• wildcards inline: /abc-{*1}

Worth delaying until we are feature complete for path routing.
Constraints will complicate things.

Will be hard to get right on first try.

See the path_tree test: repeated_single_named_param