improved type adapter declarations

[martino2k6]

Bind the type adapters at interface-level to keys, as opposed to requiring each method which uses a custom type to use the TypeAdapter annotation.

This should result in less duplication for the definitions, and should also pave the way for caching the type adapter instances when the invocation handler is created as opposed to instantiating them on each method invocation (where required) and then just throwing them away.