Gas snapshotting

Writing this down so I don't forget. Could implement [this hardhat gas snapshotting pattern](https://gist.github.com/gakonst/258a6ccd6bb752248999a13e8dd82a64) pretty easily by:

- Adding a `dapp snapshot` command that runs `dapp test` and searches through the output for `(gas: XXXXX)`, then stores the results in a `.dappshots` file (unless overriden via an env var/flag) with each test's gas usage tied to its name

- Adding a `dapp check-snapshot` command that runs `dapp snapshot` (but outputs to a temp file instead of `.dappshots`) and compares it with `.dappshots`. If they don't match the command exits with an error.