Implement local-avoidance for mobile entities

[phrohdoh]

Currently multiple entities can occupy the exact same position. This is not desired.

Resources:

• https://answers.unity.com/questions/790857/unit-local-avoidance-in-rts-type-games.html
• http://grail.cs.washington.edu/projects/crowd-flows/continuum-crowds.pdf
• https://gamedevelopment.tutsplus.com/tutorials/understanding-steering-behaviors-flee-and-arrival--gamedev-1303
• http://orhanbalci.github.io/rust-steering-behaviors/steering/index.html