asciiart: add

This ascii art library contains small functions for drawing squares and
inserting text strings into two dimensional buffers, representing a
screen.
map: add

Map contains functions to plot a map of neighbouring systems using the
asciiart functions.
map: use

Add a map command that displays a map of neighbouring systems to the
player.
universe.c: expand and clarify comment

System proximity is now somewhat limited by the resolution of the map.
If systems are too close, the map will be too cluttered.