Move grid management functions out of polygons.lua, in a new file called gridmanager.lua
Explicitly call other files and grid management functions from init.lua
to make workflow more apparent
Move mapgen loop out of init.lua, into mapgen.lua
This introduces new parameters 'map_x_size' and 'map_z_size' that default to 15K
Deprecates 'grid_x_size' and 'grid_z_size'; if they are present, corresponding
values of 'map_x_size' and 'map_z_size' are automatically written in config files.
Also rework compatibility system to better compare versions,
and bump version to 1.0.2-dev1.
