accounts/abi: fixed return tuple and string, bytes return type parsing

Removed old unmarshalling of return types: `abi.Call(...).([]byte)`.
This is now replaced by a new syntax:

```
var a []byte
err := abi.Call(&a, ...)
```

It also addresses a few issues with Bytes and Strings and can also
handle both fixed and arbitrary sized byte slices, including strings.