fix: enforce eip-2 imposed limits and secp256k1 upper bound for private keys

[davidmurdoch]

Sending transactions from an impersonated account with a "large" account number, like fffffffffffffffffffffffffffffffffffffffe, would result in the error "The nonce generation function failed, or the private key was invalid" due to the way we fake transaction signing in ganache. Previously we would take the account number plus the first 12 bytes of the account number, fffffffffffffffffffffffffffffffffffffffe + ffffffffffffffffffffffff, and would use that as a fake private key. This results in an invalid key, as secp256k1, the elliptic curve used in Ethereum cryptography, has an effective maximum private key value of 0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364140n (AKA secp256k1_n - 1, or the total number of non-trivial points on the curve). This fixes #2586.

While implementing this fix it was discovered that we were not rejecting transactions with too-high s-values; i.e., s-values that are greater than (secp256k1_n - 1) / 2. This restriction was added way back in Ethereum's first hardfork, homestead, as part of EIP-2 in order to remove the possibility of "malleable" transactions. While somewhat unrelated to the core reason for this fix, it has been added as part of this PR. This fixes #2600.

[MicaiahReid]

Reviewed! This also needs a description that is release ready.