addrs: ModuleInstanceStep.String

When ModuleInstanceStep values appear alone in debug messages, it's easier
to read them in a compact, HCL-like form than as the default struct
printing style.

addrs/module_instance.go

@@ -413,3 +413,10 @@ func (m ModuleInstance) TargetContains(other Targetable) bool {
 func (m ModuleInstance) targetableSigil() {
 	// ModuleInstance is targetable
 }
+
+func (s ModuleInstanceStep) String() string {
+	if s.InstanceKey != NoKey {
+		return s.Name + s.InstanceKey.String()
+	}
+	return s.Name
+}