[quicwg/base-drafts] VarInt all the HTTP things (#2437)

[Mike Bishop <[e-mail]>]

As discussed in Tokyo, varints all the things.  This led to a few ancillary changes along the way:
- I stole transport's frame type IANA rules and used it for our three new varint registries.  (This was alluded to in discussion in Tokyo.)
- Unidirectional stream types are now a fixed sequence starting from 0x00, rather than scattered code points that "just happen" to have ASCII values.  (This was explicitly requested in discussion in Tokyo.)
- It's impractical to enumerate and have IANA reserve the grease values for a much larger space; therefore, I shifted the requirement to saying that IANA MUST NOT allocate those code points to anyone.  (This also avoids IANA having a publicly-visible list of the grease codepoints, which is a nice perk.)
- The `0x?a?a?a?a` is less effective greasing, as it falls entirely in the private use range.  I changed everything to use the `0x1F * N` formulation, which has the side benefit of being easy to produce and less obvious to identify on first glance.  (However, only two of these values fall in the standards-assigned region.)

If any of these ancillary changes are controversial, I'm happy to split them out into separate issues for discussion, but they make these changes fit more naturally.

Fixes #2233.
Fixes #2253.
Fixes #2275.
You can view, comment on, or merge this pull request online at:

  https://github.com/quicwg/base-drafts/pull/2437

-- Commit Summary --

  * Varint the frame types
  * Concomitant change to HTTP_MALFORMED_FRAME_TYPE
  * Varint the stream types
  * Format values
  * Varint the Settings Identifiers
  * Mention the difference in HTTP/2 considerations
  * Make frame type greasing consistent

-- File Changes --

    M draft-ietf-quic-http.md (161)
    M draft-ietf-quic-qpack.md (14)

-- Patch Links --

https://github.com/quicwg/base-drafts/pull/2437.patch
https://github.com/quicwg/base-drafts/pull/2437.diff

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