sign and verify contracts with secp256k1

[littleskunk]

For a better performance on the bridge side on sending ALLOC and renew contracts.

• [x] sign contract
• [X] benchmark test sign contracts old: sign contract x 16.83 ops/sec ±5.17% (80 runs sampled) new: sign contract x 122 ops/sec ±1.54% (141 runs sampled)
• [x] verify contracts
• [x] benachmark test verify contracts old: verify contract x 21.87 ops/sec ±1.33% (98 runs sampled) new: verify contract x 632 ops/sec ±0.81% (147 runs sampled)
• [x] unit test private key padding

[littleskunk]

Review and merge please.