DEV-1096 - add a new "pipeline" concept (#67)

* add a new "pipeline" concept

- added new Pipeline type which is a series of stages
- added a global registry to facilitate plugin architecture
- 100% test coverage

* Refactor rulesets to provide and use Pipeline

* fix copypasta comments

* fix lint for unused method

* include game over stages in ruleset pipelines

* clean up unused private standard methods

* remove unused private methods in squad ruleset

* remove unused private methods in royale ruleset

* refactor: pipeline clone + return next board state

* YAGNI: remove unused Append

* refactor: improve stage names

* add no-op behavior to stages for initial state

* refactor: no-op decision within stage functions

* remove misleading comment that isn't true

* dont bother checking for init in gameover stages

* remove redundant test

* refactor: provide a combined ruleset/pipeline type

* fix: movement no-op for GameOver check

IsGameOver needs to run pipeline, move snakes needs to no-op for that

* add test coverage

* refactor: improve stage names and use constants

* add Error method

Support error checking before calling Execute()

* update naming to be American style

* panic when overwriting stages in global registry

* rename "Error" method and improve docs

* use testify lib for panic assertion

* remove redundant food stage

* use ruleset-specific logic for game over checks

* re-work Pipeline errors

* rework errors again

* add defensive check for zero length snake

* use old logic which checks current state, not next

* add warning about how PipelineRuleset checks for game over

board.go

@@ -25,7 +25,7 @@ func NewBoardState(width, height int32) *BoardState {
 	}
 }
 
-// Clone returns a deep copy of prevState that can be safely modified inside Ruleset.CreateNextBoardState
+// Clone returns a deep copy of prevState that can be safely modified without affecting the original
 func (prevState *BoardState) Clone() *BoardState {
 	nextState := &BoardState{
 		Turn:    prevState.Turn,