Integrate MultiVerb into the `servant` packages

[theophile-scrive]

This commit is Part 1 of the integration, where only the packages of this monorepo are touched.

MultiVerb is integrated alongside of the other Verb machinery, so that people may take their time to transition to it.

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IsWaiBody will not be able to live in its current form, as it mixes concepts from the Server and Client packages.

This impacts SomeResponse, which itself impacts the HasClient and HasServer instances. ResponseF can be replicated internally, even without the httpVersion field. SomeResponse can be kept on the server, but the client can move to use ResponseF.

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The IsResponse and IsResponseList classes are split between client-side Reponse{,List}Unrender and server-side Response{,List}Render.

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This is a collaborative work between Scrive AB and Wire Swiss GmbH.

[theophile-scrive]

@fisx @ysangkok I'd appreciate a review or questions if some things are not clear!

[theophile-scrive]

@pcapriotti @fisx I'd love your final feedback on this. :)

[theophile-scrive]

Very well, with this last input, I think we are good to merge.

[theophile-scrive]

@fisx would you do me the honour?

[fisx]

@fisx would you do me the honour?

merged. also the honor is all mine :-) thanks for your infinite patience!

[fisx]

@phadej @jkarni @...

any objections to me cutting a new release tomorrow? (come to think of it, should i be on the google mailing list so i can ask these things there? i remember vaguely having been opposed to gmail when that was started, but my opposition appearently didn't make google go away... :)

[tchoutri]

I'd like us to take some time to address some of the tickets that have appeared since the last release before.

[tchoutri]

Moreover I'd also be comfortable with advertising the MultiVerb feature first so that people may try it before we cut a release.