MIR: switch to BigInt/BigUint

Context

This has been a long-standing issue that we're using i128/u128 instead of bigints. We kept putting this off due to some uncertainty about which bigint implementation to choose.

We need bigints for bls arithmetic however, so about time we chose something, I reckon.

I went with num-bigint to simplify interop with the Kernel SDK.

Resolves #6410 (closed)

Based on: !10972 (merged)

Manually testing the MR

cargo test

Checklist

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