simplified namespace init

- the Context now handles setting up the environment
- the module & Sandbox can now just make a copy of it
oops, forgot about callLater in _set_volatile
fixed end-of-run reporting

- hitting esc after a non-animation no longer triggers a weird `ok
*now* the run is over) message in the output pane
reset the canvas size & bg before every run

- but not in between iterations of setup/draw/stop
- as a result you can now call size() and background() in setup() and
have the settings persist for the whole run (rather than needing them
to be at the top of draw())
libraries no longer require ximport()

- you can now use a plain-old `import` statement
- libs that need a _ctx ref can get one by calling .lib.register() in
their init