Write concepts blog post about snakes, rats and turtles

[apirogov]

The post should explain the coordinate-free representations, properties and invariants of snakes and rats, etc.

It should cover all practically interesting things relevant for tiling, so it should also explain matching and gluing.

Automatic match site discovery however could be postponed (or first done in a brute-force inefficient way).

[apirogov]

The post maybe could make use of #17 for a possible formalization of snakes - the advantage is that we do not have to introduce "oriented points" i.e. turtles, but instead simply use units and their angle with the "turtle-like interpretation"