Rewrite execTransactions

[cag]

Right now, there are a bunch of issues which while they may seem unrelated are related in a lot of ways. There's a bunch of work necessary to get this thing to always "be able to do batch transactions while simultaneously constructing a contract proxy that will stick with some account forever, and also make errors happen in an informative manner if possible."

It has to work in real scenarios (#55). For the most part, this seems to be true, as even though the test suites are failing, they aren't failing that badly. (#58)

One reason for the failing tests has to do with it currently being impossible to send safeTxGas to the CPKFactory for the first batch of transactions (#45). Doing that would help make estimating gas costs for these transactions more consistent with the already-existing-proxy use case as well, since the current gas estimation work does not do anything for proxies which don't exist yet. This has to do with the safeTxGas causing reverts if there's not enough gas available to execute the transaction, so the JSONRPC eth_estimateGas would actually generally sort of work, and actually passing a gas value to the transaction options would be unnecessary.

Actually getting a value for safeTxGas is tricky, as the value is returned as revert data from a call to requiredTxGas. Different clients handle that differently, and there would probably have to be some way of doing this for the non-existent-proxy use case. This is kinda tricky because requiredTxGas has to be issued from the proxy as well.

Solving that would require changing the CPKFactory code, which would then involve redeploying the factory, and that would probably involve all contract changes planned for the next release. Of course, we probably shouldn't just leave all v1.1.1 behind with new improvements, so the strategy for handling new proxy addresses should be hashed out (#38), though perhaps we could put this off until later.

Contract changes related to being able to send safeTxGas to the CPKFactory include:

• Gas relayer support, which would involve making the CPKFactory metatransaction compatible in a way similar to how the Safe is. (GAS relayer support  #19)
• Related to the last point is counterfactual contract support. (Allow counterfactual contract deployment. #13) At least this seems to be a change that should be done if the previous change is to be done.
• Making the deployment call on the factory payable. (Make deployment call on factory payable #39) Sort of depends on Closes #186: Make execTransaction payable safe-global/safe-smart-account#202 being released, where execTransaction is being updated in a similar way. This would allow the msg.sender to send an ETH value along with the overall transaction. Also related to the first point, in that maybe the user wants to use some ERC20 as gasToken with a relayer for their transactions.

These changes imply a major overhaul of the CPKFactory.

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Anyway, I can't do everything above at once. My plan is that this PR does the following:

• Fixes Ensure compatibility with major clients #55... somehow
• Changes gas estimation to a binary search on calls to methods that return a success bool.
• Fixes attemptTransaction error has overwritten the original error message #54 by attempting to collect and decode revert data postflight if the estimation above sees that the transaction would fail no matter the gas value up to the block limit.