Rename package manifold to 'space'
Remove the dependence to manifold in spatial index
Merge spatial index into package 'space'
Split display system into subsystems
Rearrange gfx.Sprite fields to reflect sort order

Layer is the primary sorting criterion for the sprites. Lexicographical order
is the natural ordering for structs, so putting the layer field first implies
that the sprites should be sorted according to it first.
Makefile command to run a doc server
Animation system
Split off non-mutating game world queries into their own system
Don't package unused assets with the program binary
Display fx system
Shoot action stub, tighter directional command dispatch
Animation effect for shooting