Base documentation off of compiled interface, not IContract file

[area]

Closes #1272.

Primarily this is a slight rework of the docgen.js script. Instead of just using the interface file for a contract to work out which functions should be in the docs, it uses the ABI and a flattened version of the source to generate the documentation of every function that is actually on the contracts. In the event of a function being documented in multiple places, it picks the documentation that is the longest.

To get this to work cleanly, I've ended up introducing e.g. BasicMetaTransaction is IBasicMetaTransaction and equivalents in various places, which seems like something we should have been doing anyway to ensure consistency.

NB Currently based on #1305

[area]

The main decision I've taken that I could have gone the other way on is how I've handled getMetaTransactionNonce. I've added (IBasicMetaTransaction, BasicMetaTransaction) to the override where it's relevant. This is because IBasicMetaTransaction defines executeMetaTransaction as an external function (implicitly, because it's an interface), whereas BasicMetaTransaction defines it as public (because it's used in executeMetaTransaction and we don't want an external call). Therefore, if both are inherited, we need to explicitly override both.

The alternative, I think, would be to move (most of) our interfaces to abstract contracts and do the override ourselves in BasicMetaTransaction.

[kronosapiens]

Looks good!

[kronosapiens]

📚