Investigate using types alone for Decoder bindings

[codefromthecrypt]

currently, we use method key (ex. IAM or IAM#auth(String,String)) to wire decoders to the methods that need them. This is consistent with error handlers, etc, but possibly overkill for decoders, where the intended result is always correlated to the return type of the method (or the type param of the observer).

If we switch to type-only, it is possible for us to use Set bindings, to collect decoders. Something far less complicated then our current approach.

[codefromthecrypt]

closed via #20