* Added `LinkSelectorFilter()` link option.
* Added `describedType`.
* Refactored `readComposite` to try decoding as described type when not a known composite.
* Added `describedType` to marshal tests.
* Added `String()` methods for debugging.
Note: Event Hubs was sending back `0x137000000a` as descriptor code when `LinkSelectorFilter()` was using a string as the descriptor. I haven't been able to figure out what that code refers to.
Kale Blankenship