fix!: mark unconstrained methods of runtime BN as unconstrained

[kashbrti]

resolves #133
this might be a breaking change because the unconstrained methods might have been used
outside unsafe blocks before (they were in the test module).

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• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[TomAFrench]

Hey, I closed #133 as there's actually a reason for why these methods are not marked as unconstrained

noir-bignum/src/runtime_bignum.nr

Line 24 in 1e9bd8a

// They're not actually decorated as `unconstrained` because to return the `params` (as part of Self) from an `unconstrained` fn would cause range constraints. Instead, each `__` fn wraps a call to an unconstrained fn, so that the already-range-constrained `params` can be inserted into Self after the unconstrained call.

Can you update the doc comments to direct the developer to read the documentation attached to the RuntimeBigNum struct type and include the justification in a doc comment there?

[kashbrti]

closing this following the discussion made on the issue.