accounts/abi: handle named ouputs prefixed with underscores

[karalabe]

I've found this issue while trying to wrap the smart contracts from Akasha.

If a smart contract has named outputs (i.e. all of the outputs have names), abigen will group them into an on-the-fly generated struct, instead of simply returning a gazillion return value:

// Solidity: function resolve(_node bytes32) constant returns(akashaId bytes32, addr address, donationsEnabled bool, fn uint8, digestSize uint8, hash bytes32)
func (_ProfileResolver *ProfileResolverSession) Resolve(_node [32]byte) (struct {
	AkashaId         [32]byte
	Addr             common.Address
	DonationsEnabled bool
	Fn               uint8
	DigestSize       uint8
	Hash             [32]byte
}, error) {
	return _ProfileResolver.Contract.Resolve(&_ProfileResolver.CallOpts, _node)
}

For the above example, abigen can convert the return names (e.g. akashaId) into output struct field names (e.g. AkashaId) by capitalizing the first letter. Similarly, the abi package can also unpack the fields using the same logic.

Unfortunately, Akasha doesn't name its return values like that, rather it prepends an underscore in front of all the names, resulting in the following binding:

// Solidity: function resolve(_node bytes32) constant returns(_akashaId bytes32, _addr address, _donationsEnabled bool, _fn uint8, _digestSize uint8, _hash bytes32)
func (_ProfileResolver *ProfileResolverSession) Resolve(_node [32]byte) (struct {
	_akashaId         [32]byte
	_addr             common.Address
	_donationsEnabled bool
	_fn               uint8
	_digestSize       uint8
	_hash             [32]byte
}, error) {
	return _ProfileResolver.Contract.Resolve(&_ProfileResolver.CallOpts, _node)
}

Of course trying to run the above generated code will fail at two locations:

• We can't unmarshal the returned data fields into _fieldname fields, since they are private.
• The abi package cannot match the fields with the returns, since it only capitalizes.

This PR solves the underlying issue by having a pre-processing step in both abigen and also in abi, where we discard all prefixing underscores from variable names before trying to generate output structs, as well as before trying to match the deserialized outputs into said struct.

The PR also adds a live test case (backed by a simulated backend) to verify that both binding underscores work (i.e. they compile and generate the correct code), as well as unmarshalling works (i.e. the test creates a live contract and retrieves data from it).

[holiman]

LGTM