Args -> ArgumentsNode

[davidwessman]

I tried to address #351 since it seemd limited in scope, so hopefully:

• Fixes Args -> ArgumentsNode #351
• Changed all use of parts to arguments as well.

I was not sure if the s(:args, children, location) in lib/syntax_tree/translation/parser.rb should be updated as well (tried it but it broke the tests).
And the replacement of parts -> arguments was trial-and-error in a lot of places, since it was hard to know the types of
node.contens.arguments.arguments.arguments or something similar.

Please let me know if I should change anything, or feel free to reject the changes :)