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

solo.go

@@ -1,13 +1,27 @@
 package rules
 
+var soloRulesetStages = []string{
+	StageMovementStandard,
+	StageStarvationStandard,
+	StageHazardDamageStandard,
+	StageFeedSnakesStandard,
+	StageSpawnFoodStandard,
+	StageEliminationStandard,
+	StageGameOverSoloSnake,
+}
+
 type SoloRuleset struct {
 	StandardRuleset
 }
 
 func (r *SoloRuleset) Name() string { return GameTypeSolo }
 
+func (r SoloRuleset) Execute(bs *BoardState, s Settings, sm []SnakeMove) (bool, *BoardState, error) {
+	return NewPipeline(soloRulesetStages...).Execute(bs, s, sm)
+}
+
 func (r *SoloRuleset) IsGameOver(b *BoardState) (bool, error) {
-	return r.callStageFunc(GameOverSolo, b, []SnakeMove{})
+	return GameOverSolo(b, r.Settings(), nil)
 }
 
 func GameOverSolo(b *BoardState, settings Settings, moves []SnakeMove) (bool, error) {