Name messages using CHOICE variable name enums
Name messages using CHOICE variable name enums

- Add "Define: " attribute tag, equal to "request" for request PDUs and
"choice" for top-level message unions

- Rather than building a single table of request PDUs, keyed on the PDU
definition struct (which isn't very usable), build a table per union type
which is keyed on the enum values actually contained within the struct

- Enumerate response types by the enum values contained in all "Define:"'d
union types, rather than by the PDU definition structs

cmusatyalab/minirpc,cmusatyalab/minirpc