Extract URL component parameters in a type-safe manner

[rcrowley]

This would be one thing to rip off from GoRest.

[rcrowley]

I think I have a solution! To preserve the orthogonality of Tiger Tonic features, TrieServeMux will continue to extract URL component parameters as strings into a url.Values but Marshaled will use tags like url:"foo" to pull {foo} from the url.Values into the request struct, at which point its type will have to be made to match. If the types are incompatible (for example, if foo is presented for an int field) we can respond 400.