Conflicting routes are only detected in validate() or run()

[cschreib]

The code below does not generate any exception or error in v1.0+5:

CROW_ROUTE(app, "/page1")([]{ return "Hello world 1"; });
CROW_ROUTE(app, "/page1")([]{ return "Hello world 2"; });

It is only later down the line, when calling app.run(), that we get an exception:

"handler already exists for /page1"

This is a bit late, and if the routes are defined in various places, it can be difficult to track the offending CROW_ROUTE. If the error was raised immediately when creating the route, debugging would be easier.

I appreciate there's probably a cost to pay for this check, and that not everyone wants to pay for it on each new added route. An alternative I considered was to explicitly call validate() in my code, after adding each route. But unfortunately crow::Crow only expects validate() to be called once, when all the routes are set up. This is because the validated_ internal flag is never reset to false when adding new routes. If it were, then I could just call validate() whenever I want to do error checking.

I thought this could be an easy fix, but currently validate() does more than just checking, it also sets up the static route and applies the blueprints. These are actions that shouldn't be done multiple times, but they are also not strictly speaking "validation". I think they should probably be separated from the actual route validation, perhaps into a separate function.