Update to core

Stream: mathlib4

[Wrenna Robson]

When updates are made upstream in core, when do they propagate down to Mathlib? Or, indeed, to Batteries?

[Henrik Böving]

All major community projects maintain nightly testing branches that are kept compatible with the latest nightly release (mostly by Kim) so that when we make a new Lean release/rc the respective master branches can be updated ASAP. New releases/rc's are made approximately every month somewhere around the middle of the month.

[Wrenna Robson]

RIght :)

[Wrenna Robson]

Thanks!