ethcoder refactor and introduce EncodeContractCall

[pkieltyka]

• introduce ethcoder.ABI, for managing ABIs and ABI signatures for both methods and events
• rename EventDef to ABISignature. Previously EventDef was used for decoding event definitions like, "Transfer(address,address,uint256)", however this is actually an ABI signature and it can be used by both methods and events, so it's been renamed and made more versatile to cover both methods and events
• rename many ethcoder.* method names, and move older names to abi_deprecated.go with a deprecation warning
• introduce new EncodeContractCall method, which accepts the ABI as either as a ABI method signature, or as a JSON string, and the arguments as string values to call the contract. It also supports nested encoding, please see tests to see its full capabilities. NOTE: the EncodeContractCall is similar to go-sequence's former EncodeDelayedABI method, but with a bunch of improvements