Migration to 0x coverage tools

[nventuro]

For a while now we've been having multiple issues with solidity-coverage. A bug I encountered yesterday broke the proverbial camel's back: when restoring a snapshot, account nonces are not restored, which causes issues with some deploy schemes, such as ERC1820's.

This is an effort to migrate our testing infrastructure to 0x's sol-coverage, which will hopefully be more stable. Many changes needed to be made however, since it uses web3-provider-engine under the hood. I also had to move some contracts around, since 0x's tooling requires each contract to be stored in a file named after it.

[nventuro]

This is still WIP, but I want to take a look at how long this new setup takes on Travis.

[frangio]

As of #1684 (comment), we were able to update our current coverage analysis tool to solve the bug we had ran into. It'll last a bit longer like that, but solidity-coverage being unmaintained, it will break again.

Our experience trying out @0x/sol-coverage was also kind of rough unfortunately. @nventuro can share more details about it.

[nventuro]

Most of the issues come from the fact that 0x's tooling uses web3-provider-engine, so in order to connect to a ganache node you either:
a) setup a GanacheSubprovider
b) connect to it via the RPCSubprovider

As can be seen in the test runs in this PR, option a is very slow (between 2 and 3 times slower than our current setup). It looks like the suite becomes slower as each test is executed, since running single contracts with .only takes about the same on both setups: maybe there's some sort of resource leakage going on.

Option b had other problems, and in the end we couldn't get it to work. There may be a bug somewhere in the stack, since after a couple requests the provider enters a loop where it asks for the receipt for a block that hasn't been mined yet, which it never exits.

Even if we managed to have the tests running, the coverage tools still lack some features solidity-coverage has (such as instrumenting contract creations from inside another contract, testing both branches of a require, among others). Because of this, we'll be sticking with it for now.