* beam now collides with aliens.

* two new types of aliens. only one rules.
  * removed gfx_write in favor of gfx_putchar and gfx_putstr.
  * added bonuses at alien death. they do nothing yet.
  * seeker missiles. 'b' to test.
  * weapon resolution is now 16*char. needs to be generalized.
  * fixed the supernova bugs. center could collide with the tunnel, and
    the last frame was badly displayed.
  * lots of cleanups everywhere.

* used a cool Bresenham algorithm to draw the nuke circles.
  * nuke collides with aliens.

the nuke is still ugly, almost no collision detection. Nice demo :)