Implement new, extensible CREATE, EXTEND, and related cells

Proposal 200 specifies new, extensible CREATE, EXTEND, and related cells. Nick thinks we should implement it really soon and suggests Andrea as tech lead.

changed milestone to %Tor: 0.2.4.x-final

added SponsorZ component::core tor/tor milestone::Tor: 0.2.4.x-final priority::medium resolution::implemented status::closed tor-relay type::project labels

Trac:
Milestone: N/A to Tor: 0.2.4.x-final
Keywords: SponsorZ deleted, SponsorZ tor-relay added

Trac:
Cc: andrea, nickm to andrea, nickm, mikeperry

This is implemented in my branch "ntor", which needs far more review and testing. See also #7202 (moved) .

Trac:
Status: new to needs_review

By way of code review, I'm attaching my notes for this and 7202. A lot of the things that popped out to me turned out to be addressed in later patches, but they still raised questions I felt like I should know, so I'm mentioning them all here.

Trac:
andrea_7199-7202_notes.txt

Merged.

Trac:
Resolution: N/A to implemented
Status: needs_review to closed

closed

moved to tpo/core/tor#7199 (closed)