Add a simple constructor for Wallet

[moh-eulith]

Motivation

I implemented Signer, but realized that I had to copy a large amount of code (Wallet and more) just to get a working implementation.

Solution

A plain constructor for Wallet allows other implementation of Signer to use this code.

PR Checklist

• Added Tests: correctness is by inspection of the one liner
• Added Documentation: yes
• Updated the changelog: I don't understand the changelog. It's from a long time ago?

[gakonst]

Can you give an example of when you had to implement your own DigestSigner? This doesn't make full sense to me on why it'd be needed

[moh-eulith]

There are devices (and custodians and what not) out there than can do ECDSA signatures: given a hashed message, spit out a 64 byte signature from a secretly held key.

Of course having a raw signature is far from being able to talk to the Eth chain. Wallet and associated code do all that heavy lifting.

Honestly, the question seems a bit odd given that the code is already generified over the digest signer, for the very practical reason that there are 2 different implementations even in the library itself (private key and yubi key).

[gakonst]

Hmm I was thinking that you could do this by implementing a Signer and then the SignerMiddleware does all the lifting, but I can see the need to expose a constructor anyway