Add checked arithmetic methods to U256 and I256 #423

Job	Run time
collect-examples	18s
test-example (events)	2m 30s
test-example (increment_with_fuzz)	3m 34s
test-example (increment)	2m 30s
test-example (cross_contract/contract_a)	2m 27s
test-example (cross_contract/contract_b)	2m 53s
test-example (custom_types)	2m 31s
test-example (increment_with_pause)	2m 57s
test-example (bls_signature)	2m 30s
test-example (multisig_1_of_n_account/contract)	2m 25s
test-example (auth)	2m 25s
test-example (pause)	2m 26s
test-example (mint-lock)	2m 27s
test-example (import_ark_bn254)	2m 29s
test-example (privacy-pools)	3m 17s
test-example (timelock)	2m 22s
test-example (groth16_verifier)	2m 33s
test-example (upgradeable_contract/new_contract)	2m 19s
test-example (deep_contract_auth)	2m 54s
test-example (other_custom_types)	2m 19s
test-example (liquidity_pool)	2m 28s
test-example (errors)	2m 34s
test-example (token)	2m 25s
test-example (single_offer)	2m 24s
test-example (logging)	2m 24s
test-example (fuzzing)	3m 21s
test-example (deployer/deployer)	2m 58s
test-example (privacy-pools/contract)	3m 17s
test-example (ttl)	2m 23s
test-example (merkle_distribution)	2m 19s
test-example (simple_account)	2m 28s
test-example (eth_abi)	2m 29s
test-example (alloc)	2m 27s
test-example (hello_world)	2m 36s
test-example (account)	2m 26s
test-example (atomic_multiswap)	2m 44s
test-example (upgradeable_contract/old_contract)	3m 4s
test-example (deployer/contract)	2m 22s
test-example (atomic_swap)	2m 19s
	1h 39m 34s

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