Split executeMessage into executeCall, executeCreate

[s1na]

This PR:

• Brings all state snapshotting-reverting to executeMessage and moves call or create specific logic to _executeCall and _executeCreate. I think the differences between the two are more than the similarities, and it warrants two separate methods. It also prepares for moving call/create logic from EEI to Interpreter
• For now _executeCreate is a relatively big function, this will be further broken down in future
• It also changes how create collisions are handled. Previously reset the code to an invalid code which would throw. It took me a while to understand how exactly this was working. Cool trick, but confusing. So it now directly returns an error instead
• Also re-orders functions and moves _reduceSenderBalance and _addToBalance to the bottom (they'll have to eventually be removed and moved to stateManager or something similar)

(To be merged into #479)

[holgerd77]

Just as a note: this still includes the commits from the previous PR.

[holgerd77]

Apart from the repeated commits: directly did the review, looks great, Interpreter is beautifully readable now, also had some laid back all-file look on this to get a bit the broader picture.

Feel free to merge, thanks Sina.