perf: inline Decidable instances

[hargoniX]

This PR inlines several Decidable instances for performance reasons.

Unlike the previous #10934 it does not attempt to also simplify the Decidable instance system as
that has proven to have non trivial performance impact.

[Rob23oba]

I don't really think this is performance difference but rather a bug in the instance size calculation (see #11009)

[hargoniX]

That's good to know, I would just like to avoid people hitting this bug so we'll do the revert and tamer change first and if the instance size is eventually fixed we can go back again.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries CI can not be attempted yet, as the nightly-testing-2025-10-28 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Batteries CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-29 09:12:00)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2025-10-28 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2025-10-29 09:12:01)

[hargoniX]

Works with mathlib: leanprover-community/mathlib4-nightly-testing#101