Final example tidying

[rparrett]

Objective

Just some further tidying.

Solution

• Do serialization and blocking IO in IoTaskPool
• Organized setup into a few logical groups of systems
• Added some grass for the fox to run on and cast shadows on, and other minor aesthetic scene tweaks
• Organized imports -- std, bevy, third party
• Organized overall structure -- imports, statics, fn main, resources / components, systems, other types and impls
• Further simplified background/border color definitions
• Other minor bits of tidying

[pcwalton]

Merged, thanks!