Make naming consistent for definitions

[schrockn]

We need to do a pass on naming definitions. At minimum inputs and outputs should consistently by input_defs and output_defs. This is mostly because python is dumb and has "input" as a keyword which no one ever uses. So you are either stuff with input_ or input_def or similar. Probably all of these things (solids, arguments, contexts) should be suffixed with def by default but I do still react negatively to that.