accounts/keystore, crypto: enforce 256 bit keys on import

accounts/keystore/key.go

accounts/keystore/keystore.go

accounts/keystore/keystore_passphrase.go

accounts/keystore/keystore_passphrase_test.go

accounts/keystore/presale.go

core/types/transaction_test.go

crypto/crypto.go

internal/ethapi/api.go

swarm/api/config_test.go

tests/util.go