Support for marking a server as deprecated

Since OpenAPI supports marking an operation as deprecated, we're working up a little feature to be able to flag Servant servers as deprecated in a way that will be reflected in the generated docs, used like so:

```
type Admins =
  Deprecated :> "users" :> "admins" :> GetAdmins
```

If this is the appropriate package, would there be any interest in accepting a PR?