move transfer-coding ABNF to [Semantics]?

[reschke]

This is the remaining ABNF production that Semantics imports from Messaging (for the definition of "TE"). Maybe we should move the production to messaging, reversing the dependency.

[mnot]

Discussed on call; worth a try.

[reschke]

...turns out that there are a few more things to consider:

• Semantics currently claims there are regular parameters here, but transfer-parameter is special wrt BWS
• should we move the special case of "trailers" in the grammar into prose? In any case, what would it mean to have a ranking on "trailers"? Invalid, thus ignore?
• "t-ranking" seems to duplicate "weight"