chore: don't expose Shrink

[plp127]

Mark Shrink and equivShrink as @[no_expose]. This ensures downstream files using the module system can't take advantage of the specific definition of Shrink as an application of Classical.choose.

See also #general > Cardinality model incompatible with Lean compiler @ 💬 on Zulip for an example of bad things that can happen if Shrink is exposed.

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[github-actions]

PR summary cb6b2796fb

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[plp127]

Maybe we could also have an opaque shrinkImpl (α : Type v) [Small.{w} α] : { T : Type w // α ≃ T } so that downstream files not using the module system also can't take advantage of the definition of Shrink as a Classical.choose.

[vihdzp]

I don't know if there's precedent for opaque defs in Mathlib, but no_expose is an improvement in either case.