Add "EliminatedBy" to snake eliminations. (#11)

* add eliminated by

* add test

* make sure largest snake is listed as eliminator on head to head collisions

* remove unused type def

* Reduce memory usage during elimination checks.

Co-authored-by: Daniel Steuernol <dlsteuer@gmail.com>

ruleset.go

@@ -17,6 +17,7 @@ type Snake struct {
 	Body            []Point
 	Health          int32
 	EliminatedCause string
+	EliminatedBy    string
 }
 
 type BoardState struct {