DEV-1761: Clean up Ruleset interface

[robbles]

Removes and simplifies some of the Ruleset and RulesetBuilder interfaces, and integrates them into the CLI. This fixes some error handling in the CLI as well.

Ruleset now looks like this:

rules/ruleset.go

Lines 3 to 13 in 2766ad7

	type Ruleset interface {
	// Returns the name of the ruleset, if applicable.
	Name() string
	// Returns the settings used by the ruleset.
	Settings() Settings
	// Processes the next turn of the ruleset, returning whether the game has ended, the next BoardState, or an error.
	// For turn zero (initialization), moves will be left empty.
	Execute(prevState *BoardState, moves []SnakeMove) (gameOver bool, nextState *BoardState, err error)
	}

This is a little less friendly than the old multiple method interface, but I'm planning on adding some helpers in the same release to make it simpler to take a ruleset and run a game one turn at a time.

And RulesetBuilder takes an explicit ruleset name through the NamedRuleset to construct one of the standard rulesets.

rules/ruleset.go

Lines 77 to 104 in 2766ad7

	// NamedRuleset constructs a known ruleset by using name to look up a standard pipeline.
	func (rb rulesetBuilder) NamedRuleset(name string) Ruleset {
	var stages []string
	if rb.solo {
	stages = append(stages, StageGameOverSoloSnake)
	} else {
	stages = append(stages, StageGameOverStandard)
	}
	switch name {
	case GameTypeStandard:
	stages = append(stages, standardRulesetStages[1:]...)
	case GameTypeConstrictor:
	stages = append(stages, constrictorRulesetStages[1:]...)
	case GameTypeWrappedConstrictor:
	stages = append(stages, wrappedConstrictorRulesetStages[1:]...)
	case GameTypeRoyale:
	stages = append(stages, royaleRulesetStages[1:]...)
	case GameTypeSolo:
	stages = soloRulesetStages
	case GameTypeWrapped:
	stages = append(stages, wrappedRulesetStages[1:]...)
	default:
	name = GameTypeStandard
	stages = append(stages, standardRulesetStages[1:]...)
	}
	return rb.PipelineRuleset(name, NewPipeline(stages...))
	}

rules.Settings has also been converted into a generic set of string key-value pairs, with maps and pipeline stages now parsing out settings at runtime. This loses some type safety and adds some redundant parsing, but allows us to add new settings without them being coupled to the API. The standard set of settings exposed through client requests is preserved through client.RulesetSettings with a conversion helper.
https://github.com/BattlesnakeOfficial/rules/blob/2766ad70f4ff1ccc399539951c0972b922b9cf21/settings.go