Cleanup internals so that we don't need Signer

[ChaoticTempest]

The Signer type isn't necessarily needed to be apart of the internals. This was originally there to first get everything to work as intended, but having a dependency on this type from near-primitives without any tests can break things when we go to update dependencies. So either add tests or clean up the internals. I'd vote for cleaning up internals, since I can see this snowballing where we'd have to eventually clean it up and adding further technical debt down the line with it being used more and more.

Note that cleaning up the Signer type will require us grabbing the SignedTransaction::from_actions function and all related code dependencies which could be a lot.

[frol]

@ChaoticTempest Does #329 address your point here? To be honest, I don't see any problem with using Signer in the first place.

[ChaoticTempest]

@frol as far as I remember, yes it should accomplish this ticket. The main reason why I think I wanted to move away from it was because SignedTransaction::from_actions was/is apart a test_utils file in nearcore and seemed not so great to be using it directly just in case it got removed