* 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
