Add update_many/2

[dvic]

As discussed in #2362 (comment)

TODO

• tests
• deprecation warning message
• test on a real project
• documentation

[dvic]

@josevalim This should be now ready for review. I tried to minimize the amount of changes in this PR but render_pending_components made this really difficult (without introducing a lot of duplicate code). To be honest I was surprised once the tests passed 😅 Probably a good idea to try it out in a real project as well, haven't done that yet.

[dvic]

Oops, the CI is failing on the warning message. Should I do this differently? A message at runtime? Not at compile time?

edit: went for the runtime route for now

[josevalim]

@dvic I have been thinking about this. Instead of passing a list of assigns and a list of sockets, maybe it makes more sense to pass a list of [{assign, socket}]?

[josevalim]

Continuing in #2771.