Revamp API

[wch]

• ShinySession -> Session
• ShinyApp -> App
• reactives submodule -> reactive
• Move UI components to ui_toolkit submodule
• ReactiveVal -> shiny.reactive.Value
• ReactiveValues -> shiny.reactive.Values
• input["x"] -> input.x (and can still use input["x"])
• input.x -> input.x()
• Add input and output to server function
• output("foo") -> output() (infer name from def foo)
• Add render_text
• Always require render function for output()
• observe -> shiny.reactive.effect ?
• reactive -> shiny.reactive.calc ?
• Rename reactive.Values to shiny.session.Inputs (and also export as shiny.inputs)

Stuff for the future, in other PRs:

• Add support for if "n" not in input: and maybe if not input.n:
• Make Inputs read-only.
• Figure out good pattern for declaring types for Inputs. Maybe class MyInputs(Inputs):?
• Add class for input IDs, so that ns() isn't necessary?
• Move render_* functions to render. submodule?
• Magically type input IDs?

I've also updated the example myapp to reflect the new API: https://github.com/rstudio/prism/blob/api-change/examples/myapp/app.py
(Before merging, I'll update all examples.)

[cpsievert]

This feels slightly verbose/inconsistent to me, perhaps it could be @render.plot(...)?