Type-safe URL templates for Nancy

The Nancy web framework parses URLs for you, and passes requests under a given URL route and HTTP method to your program. It relies heavily on C# dynamic objects; these don't translate well in F# code, and it's easy to make mistakes that only show up at run time. Here we define one F# record type for each route, with an attribute that defines the URL, with placeholders for each field in the record. These types implement one generic interface for each HTTP method they support: for GET, PUT and POST, these generic interfaces also specify the types of the HTTP request and response bodies.