I've kinda given up on this as I can't seem to make construct behave
how I think it should.. The ifthenelse isn't bidirectional, and not
sure how to fix it..

* Split UAC from MIDI
* Split UAC into UAC1, UAC2, UAC3 as descriptors and
  other constants are often defined differently

This allows for downstream descriptor emitters to add indexed strings.

fixes #15

Change report_enum argument to report_prefix to clarify that it should
be a usb_protocol.types.descriptors.hid.HIDPrefix enum element.

De-plural-ify HIDPrefixes to match common Enum naming conventions

Renamed to clarify that the HIDDescriptor convenience functions add
report items, not full reports.