perf: teach borrow inference about arrays

[hargoniX]

This PR informs the borrow inference that if an Array is borrowed and we index into it, the value we obtain is effectively a borrowed value as well. This helps improve the ABI of operations that recurse on linked structures containing arrays such as tries or persistent hash maps.

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for aa4e459 against fb1eb9a are in. Significant changes detected! @hargoniX

• ✅ build//instructions: -32.0G (-0.26%)

Large changes (3✅)

• ✅ compiled/parser//instructions: -323.2M (-0.85%)
• ✅ compiled/phashmap//instructions: -259.3M (-2.65%)
• ✅ elab/big_do//instructions: -196.1M (-0.82%)

Medium changes (1✅)

• ✅ elab/cbv_arm_ldst//instructions: -1.2G (-1.54%)

Small changes (189✅, 2🟥)

Too many entries to display here. View the full report on radar instead.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase fb1eb9aaa760368d64c80a38ebca4825abed916d --onto 4bf7fa7447eea00cecba8327bb9c9e5f4485f0a7. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-23 17:16:54)

[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 fb1eb9aaa760368d64c80a38ebca4825abed916d --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-23 17:16:56)