Add Service.transform

[thosakwe]

Should be very similar to Stream.transform. The goal is to have a simple, clean wrapper around (de)serialization.

var service = app.findHookedService<MongoService>('api/users').inner;
var userService = service.transform(UserSerializer.fromMap, UserSerializer.toMap);

// Find a user, without having to do the cast yourself, etc.
var me = await userService.read('<some-id>');
print(me.balance);