DEV 1247: Add a new map generator interface (#71)

* reorganize code

* first draft of map generator interfaces

* add explicit random interface to board helpers

* implement standard map

* rename Generator to GameMap

* allow initializing snakes separately from placing them

* add random number generator to Settings

* updates to GameMap interface

* add helpers for creating and updating BoardState with maps

maps/registry.go

@@ -0,0 +1,35 @@
+package maps
+
+import (
+	"fmt"
+
+	"github.com/BattlesnakeOfficial/rules"
+)
+
+// MapRegistry is a mapping of map names to game maps.
+type MapRegistry map[string]GameMap
+
+var globalRegistry = MapRegistry{}
+
+// RegisterMap adds a stage to the registry.
+// If a map has already been registered this will panic.
+func (registry MapRegistry) RegisterMap(id string, m GameMap) {
+	if _, ok := registry[id]; ok {
+		panic(fmt.Sprintf("map '%s' has already been registered", id))
+	}
+
+	registry[id] = m
+}
+
+// GetMap returns the map associated with the given ID.
+func (registry MapRegistry) GetMap(id string) (GameMap, error) {
+	if m, ok := registry[id]; ok {
+		return m, nil
+	}
+	return nil, rules.ErrorMapNotFound
+}
+
+// GetMap returns the map associated with the given ID from the global registry.
+func GetMap(id string) (GameMap, error) {
+	return globalRegistry.GetMap(id)
+}