Redesign how protocols are declared

[io7m]

Right now, creating a new version of a protocol involves re-declaring all of the messages again:

[record A]
[record B]
[record C]
[record D]

[protocol D
  [version 1 A B C]
  [version 2 A B C D]
  [version 3 B C D]
]

When the number of messages is large, this is unwieldy. It might be better to introduce the concept of protocol expressions that are evaluated to yield a final protocol. Nothing needs to change with regards to type checking, code generation, or otherwise.

[record A]
[record B]
[record C]
[record D]

[protocol D
  [version 1
    [introduce A]
    [introduce B]
    [introduce C]
  ]
  [version 2
    [introduce D]
  ]
  [version 3
    [remove A]
  ]
]