fix: make name mangling unambiguous

[Rob23oba]

This PR fixes name mangling to be unambiguous / injective by adding 00 for disambiguation where necessary. Additionally, the inverse function, Lean.Name.unmangle has been added which can be used to unmangle a mangled identifier. This unmangler has been added to demonstrate the injectivity but also to allow unmangling identifiers e.g. for debugging purposes.

Closes #10724

[leanprover-community-bot]

Mathlib CI status (docs):

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• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase f9e140838e5fb12f0726de36bcdf451eb08559fe --onto 705dac4f77aa60d749ee2b37d102518d772d6e53. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-10 14:34:52)
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[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase f9e140838e5fb12f0726de36bcdf451eb08559fe --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-09 18:39:28)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 56f3ca6fc74ad5bfbc0feafc4d5a48e87571db62 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-21 20:16:17)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 63c06725bbe73c7316831c35e3e8d7c0e4e9be12 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-22 22:06:12)

[hargoniX]

Great work! Just a few conceptual remarks regarding whether we really want as much proof in here at this point in time.

[hargoniX]

This is a bit odd, the change failed to compile in merge queue but works here. @Rob23oba could you rebase on master to see if the issue reproduces in the PR CI?

[Rob23oba]

Hmm I hope it's not non-deterministic...