feat: SIntX as native ABI types

[hargoniX]

This PR adds SIntX types natively to our ABI. This is a slight breaking change for the FFI if you are using the SIntX types.

[hargoniX]

!radar

[leanprover-radar]

Benchmark results for 64b3217 against 8ff5b8e are in! @hargoniX

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8ff5b8ec2cf755ddaa0b8cebdfb8f9ed65c0364e --onto 3a42ee0c3060e2a8365927d205ead5990366053d. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-28 15:08:50)

[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 8ff5b8ec2cf755ddaa0b8cebdfb8f9ed65c0364e --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-28 15:08:52)

[awson]

I plan to do some ffi stuff and wonder if this patch going to land anytime soon?

[hargoniX]

Yes, we've also got more FFI changes planned in the future.