Rename args to arguments

[stasm]

The fields of our AST nodes are usually spelled out as full words. The only exception being the id, which I think is common enough to almost function on its own as a word, and args. This PR renames args to arguments, to match other names such as attributes, annotations etc.

For implementations, this has both upsides and downsides. In Python, we've seen how args is a special attribute of Exception which cannot be anything else than a tuple. In JavaScript, arguments may not be used as a variable binding in the strict mode. This would make destructuring assignment, such as in let {id, arguments} = refNode; impossible. The workaround is to use refNode.arguments directly, or to reassign the property: let {id, arguments: args} = refNode.