[RFC] Allow overlap between static routes and wildcards

[jodersky]

This changes the DispatchTrie to allow overlapping wildcard segments in paths with static ones, with a preference for the latter.

For example, consider the following routes:

@cask.get("/settings")
def settings() = "settings"

@cask.get("/:id")
def user(id: String) = s"user $id"

This is currently not allowed. With these changes, it would be allowed, and the static route settings would be preferred, with a fallback to the dynamic route user:

GET /settings => settings
GET /foo => user foo
GET /bar => user bar

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The reason I'm proposing this change is mostly for use in HTML applications (i.e. not RPC-style JSON APIs). In this scenario, short URLs are useful, since users may type them directly and associate meaning to them.

Consider for example the way GitHub structures URLs. If github were written with cask's current routing logic, it would not be possible to have URLs such as /settings and /com-lihaoyi, and instead some namespacing would need to be introduced (e.g. /orgs/com-lihaoyi) to separate these, which might not actually be relevant for users.

Of course, with these changes we will no longer catch developer errors that accidentally define wildcard-overlapping routes with non-wildcard ones. It will also be up to the application developer to make sure that there aren't any accidental overlaps between valid values of wildcards and static routes (e.g. in the example above, the application developer would somehow need to make sure that there isn't a user called "settings" in their system).

Given these drawbacks, I'd like to hear your thoughts on this in general. Personally I think that it's useful to enforce non-overlaps for API purposes, because this forces you to design a more robust url scheme. However, for HTML application purposes, I think that allowing shorter URLs is useful and probably outweighs the current limitations. Maybe we could also come up with a way to make this configurable.

[lihaoyi]

I think this looks reasonable. Let's give it a few days to see if anyone else has any views before merging it