Bring HTTP/2 specs in-house

[fasterthanlime]

In the time it took me to work around the problems in the existing h2spec + junit reporters, I probably could've ported all the tests cases to this codebase.

The implementation feels at a point where it's safe to do that. Note that the spec will definitely depend on some of fluke's own types, but that's fine.

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edit: oh also, let's use the current RFCs this time around, ie. RFC 9110-9114.

(And 7541 for hpack I suppose).