Functions to get various kinds of input
from the user:

  `input(prompt)` -- print prompt, and
          return user-entered string

  `key.available` -- returns 1 if there
          is a key press in the buffer
  `key.get` -- waits for and returns a
          single key
  `key.clear` -- clear the key buffer
  `key.pressed(k)` -- returns 1 if key
          k is currently pressed

  `mouse.x` -- mouse X position, in
          pixels from left
  `mouse.y` -- mouse Y position, in 
          pixels from bottom
  `mouse.button(which=0)` -- returns
       1 if mouse button is pressed
