Nested Route 404 Behavior change proposal

[Matt Parsons]

When you define a nested route, it is greedy - if the route prefix matches, but then the route fails to match, it will send a 404. This means that each prefix must be unique, or later ones will never trigger. Example:

```

/  IndexR POST GET

/foo: FooR

    / FooIndexR POST GET

    /#{FooId} ShowFooR POST GET

/foo: OtherFooR

    /test TestFooR

    /neat NeatFooR

```

If you request to `/foo/neat` then this will always fail, because the first `/foo` capture works and commits to that subset of the routing structure.

I want to change the default behavior of this so that we fall through on nested routes. With this change, requesting to `/foo/neat` would succeed - after `FooR` dispatch doesn't get a match, we will then continue trying the next things in the route.

Why do this? It supports incrementally introducing nested routes that can be divided by team. Consider an original structure:

```

/ IndexR 

/foo/ FooIndexR

/foo/#FooId FooShowR

/foo/neat FooNeatR

/foo/test FooTestR

```

Suppose that we have two teams responsible for these routes. We want to split the routes based on team ownership. With fallthrough, we can do that - `FooR` is owned by one team, and `OtherFooR` is owned by the other team. Indeed, we can do this incrementally:

```

/ IndexR 

/foo: FooR

    / FooIndexR

    /#FooId FooShowR

/foo/neat FooNeatR

/foo/test FooTestR

```

With fallthrough, this works. We can segment a single route out. Without fallthrough, this *also* fails.

The cost is that a true 404 will now have to go through the entire routing table, where now it will terminate with 404 more quickly.

While I can make this potentially customizable, right now it relies on linear pattern matching on the route structure to know whether or not fallthrough is reasonable or not. If I get around to making a trie-based router for yesod, then this will not easily support the no-fallthrough behavior.

Posting here to get some insights from yesod users about what they'd like to see here