Arguments as structs?

[nikis05]

Currently arguments are represented as a tuple of name and value. I think they should be refactored as structs.
Arguments:

1. Convenience of use, less re-typing same type signature. Named fields provide a more readable notation;
2. Improved consistency with the rest of API, where most of the nodes are represented as structs or enums;
3. Being able to read the position of an arg, which is currently impossible.

If you find it reasonable I can do a PR