prop224: Implementation of ESTABLISH_INTRO cell
This set of commits implements the functionality specified in p224 sections 3.1.1, 3.1.2, and 3.1.4 (note that there is no 3.1.3).
The code currently lacks unit tests for both the legacy versions (which were never tested in the first place) and for the p224-specific versions, so it's probably best to wait on a merge until I have had the chance to write a testing suite.
You can view the code at https://github.com/alec-heif/tor/tree/alec_p224_3_1
Comments welcome! Some of the functionality was already run by nickm previously, but other parts are new.Changes are separated into 9 commits.