Add multihop swap support

[busimus]

Added support for multihop swaps. It consists of multiple parts:

1. Basic client-side router to suggest possible swap routes.
2. Long form order encoder to build transactions for multihop swaps based on the chosen route.
3. View-only contract CrocMultiImpact, which is used evaluate a multihop swap in one eth_call.
4. An alternative swap class CrocSmartSwapPlan capable of utilizing all of the above while retaining all the usual swap features (slippage, fixed input or output, surplus).

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR