Extract parser into its own phase

[allanrenucci]

Motivations:

• Reuse phase infrastructure (e.g. -Ystop-after:parsing)
• Make it easier to replace the parser (useful in the REPL for example)

[Blaisorblade]

Hey! We’re trying to shrink the PR queue! (Progressed/merged on all my PRs) Do you plan to resume this, or can we close it for now?
(I agree this would be nice BTW)

[biboudis]

I believe that since this is a work in progress it shouldn't be lost, so no closing. But @allanrenucci if you have free time to write some guidelines for someone to take over that would be amazing! Merci!

[allanrenucci]

To move this forward, one would need to investigate the test failures. The dottydoc test failures should be relatively easy to fix. The cyclic and pickling errors on the other hand will require more expertise.

As a first step, one could try to extract the changes in 1b4be4b into a separate PR.

[biboudis]

I will try to take over @allanrenucci

[b-studios]

@biboudis we are again at shrinking the PR queue :) Any suggestions on how to proceed with this PR?

[biboudis]

I guess rebasing and trying to make the tests work from the start. I started from pickling errors back then, but I think this needs a new round of debugging.

[OlivierBlanvillain]

I suggest closing and reopening if someone want to work on it. The last commits are from 2018 so I'm not sure rebasing would even be a sensible option here...