[zk-token-sdk] Potential overflow when gens_capcity for the bulletproof generators become too large (ottersec #6)

[samkim-crypto]

Problem

When gens_capacity becomes too large when generating BulletProofGens, then this could panic due to overflow.

Note: For token2022 application, we only use u64/u128/u256 batched range proofs, which can't panic, but it would be good to fix a potential panic in case range proof is used elsewhere more generally.

Proposed Solution

Introduce a maximum cap on gens_capacity.