Unify and flatten Input interfaces and types

[karimodm]

Removed intermediate interface IndexedUTXOReferencer, which could be possibly implemented only by a concrete UTXO input type, in favor of a generic Input interface for which all inputs, including context inputs, are implementations of.
Some methods have also been renamed to become more consistent to what result you are expected to obtain: if you are getting the OutputID of an UTXO output its getter is then called OutputID(), if instead you need to fetch the generic identifier for an Input, no matter what input type this is, then the StateID() getter is provided, with its companion Type() to discern the concrete input contained in the interface.