Porting files from core

Stream: mathlib4

[Eric Wieser]

Should we be showing files in core on the dashboard too? This is trivial to add, but it adds 90 "unported" files to the dashboard

[Eric Wieser]

(https://github.com/leanprover-community/mathlib-port-status/pull/21)

[Eric Wieser]

Thoughts @Mario Carneiro, since you mentioned something about core files in a recent porting meeting?

[Eric Wieser]

I've turned these on on the #port-dashboard. I think we have a mixture of:

• Ad-hoc ports
• Pre-header ports
• Files that make no sense to port
• Files that we just forgot to port

[Eric Wieser]

#5945 makes it easier to actually (re)port these