This PR makes theorems opaque in almost all ways, including in the
kernel.
Already now, because of proof irrelevance, theorems are almost never
unfolded. Furthermore, the import handling allows conflicting theorem
declaration with same type and different values. This is sound, but
would be confusing if the value, and thus the import order, matters for
completeness.
So with this change, a `theorem` becomes more like an `opaque`: It has a
value (for soundness), but it is never unfolded during reduction or type
checking. There are still some places in meta code that have to peek
into theorems (e.g. `FunInd`, wfrec processing), but these are code
transformations, not reduction.
One place where reducing proofs is necessary is reducing `Acc.rec`
eliminating into Type. With this change, all proofs that need to be
reducable that way have to be `def`, not `theorem`. This is already the
case due to the module system. This does not affect uses of `Acc` via
well-founded recursion, because that has already been made opaque in
#5182. This moves the reduction behavior of `Acc.rec` further into the
“supported by the theory but not relied upon by regular Lean“ corner.
Fixes#12804
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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