Move world functions into dedicated file
Consistently call it the 'map'

It is a bit confusing to refer to parts as the 'world' and others as the
'map'.
Compute map level
Create world struct

Create a struct to encapsulate world state.  For now, it simply contains
the map.
Pretty-print map

Convert map back to string that looks like a normal ASCII map for
printing.
Ignore generated files