witness: only include elements if there's enough gas for it

This PR does changes to satisfy a new requirement when adding elements to the execution witness.

The new requirement involves only adding elements to the witness if there's sufficient gas for it.

Examples:

Imagine a JUMP that jumps to a new code chunk, charging 200 gas to be included in the witness. Before this PR, TouchCodeChunksRangeAndChargeGas(...) would add the code chunk to the witness and return the cost to be charged. If we had, say, 100 gas available, this would mean that the user didn't have enough funds to pay for that (i.e: 100<200). But it's too late, and it was already added to the witness.
Imagine an instruction writes a new balance in a non-existen address, thus this address should be created. This involves writing BASIC_DATA and CODEHASH of this address. Before this PR, we always added BASIC_DATA and CODEHASH and returned the cost. If the user only had gas to pay for BASIC_DATA but not CODEHASH, it would be too late since we have already added both.

More generally, if you have X of available gas and include something in the witness cost Y, with Y>X, then we must not include it in the witness. This is made more apparent when we try to include a wide range of code chunks and only have gas for one but are asked to include 15. Before this PR, we were including 15, not only one.

From a stateless client perspective, being this strict is fine. Knowing if you have enough gas to include something in the witness doesn't depend on what you received from the witness (i.e: this PR including less things in the witness doesn't affect it). This is because everything you need to know to be sure that you can add something new to the witness is the access witness state of which branches and code chunks were added.

gballet / go-ethereum

witness: only include elements if there's enough gas for it #495