This PR adds the `@[expose]` attribute to many functions (and changes
some theorems to be by `:= (rfl)`) in preparation for the `@[defeq]`
attribute change in #8419.
This PR adjusts the experimental module system to not export the bodies
of `def`s unless opted out by the new attribute `@[expose]` on the `def`
or on a surrounding `section`.
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Co-authored-by: Markus Himmel <markus@lean-fro.org>
This file was upstreamed from batteries; I just got bitten by the
invalid reference and it took quite a while to figure out that this one
had been moved!
