ABI Encoder v2 + ABI Spec v6.6

[gakonst]

Motivation

Partially fixes #5. Some ABI Encoder v2 structs are now supported. More complex ones such as DyDx's contracts are still not supported because they require implementing Tokenize and/or Detokenize for types such as (Address, Address, (Address, Address)) (i.e. nested tuples).

This PR breaks compatibility with ABIs that do not have a stateMutability key in their functions.

(ethabi did not support ABIs which did not include the deprecated constantkeyword. Notably, Vyper produces deprecated ABIs as well.)

Solution

1. Fork ethabi and cherrypick the useful changes I've seen across various PRs. Details in the recent commits: https://github.com/gakonst/ethabi/commits/master
2. Expand tuples in codegen, similar logic as here: https://github.com/graphprotocol/solidity-bindgen/blob/master/solidity-bindgen-macros/src/abi_gen.rs#L89-L97.
3. Re-enable all the more complex Tokenize implementations
4. Add example for generating bindings: cargo run --example abigen <contract name> path/to/contract.sol will print the bindings to stdout. If you specify an additional path, the generated bindings will be written to the file corresponding to that path