Create a transaction type that validates signature conforms to EIP-2

[Wodann]

Per EIP-2, signatures with high s values are invalid Ethereum protocol signatures and shouldn't be used in transactions, as of the Homestead hardfork.

Currently, we don't validate this upon creation of a SignedTransaction, resulting in a k256 error when recovering the address of the signature. For ease of debugging, it should be illegal to construct a SignedTransaction in this invalid state.

We propose introducing two separate structs:

1. struct for deserializing transactions with "unvalidated" signatures
2. struct for validated signatures: SignedTransaction

The SignedTransaction should have a fallible constructor which validates an "unvalidated" signature, as well as an unsafe infallible constructor which accepts an "unvalidated" signature, given the function caller's guarantee that they have pre-validated the signature.

An example of the latter use cases is when signing a transaction, as k256 pre-normalises the signature.

Definition of Done
Split the deserialization logic of SignedTransaction to a separate struct and add a safe and unsafe constructor to SignedTransaction - according to the above specification