start using range-over-func

It so happens that Environ.Each already conforms to iter.Seq2.

expand/param.go

@@ -415,11 +415,10 @@ func (cfg *Config) varInd(vr Variable, idx syntax.ArithmExpr) (string, error) {
 
 func (cfg *Config) namesByPrefix(prefix string) []string {
 	var names []string
-	cfg.Env.Each(func(name string, vr Variable) bool {
+	for name := range cfg.Env.Each {
 		if strings.HasPrefix(name, prefix) {
 			names = append(names, name)
 		}
-		return true
-	})
+	}
 	return names
 }

interp/api.go

@@ -810,10 +810,7 @@ func (r *Runner) Run(ctx context.Context, node syntax.Node) error {
 		r.setErr(NewExitStatus(uint8(r.exit)))
 	}
 	if r.Vars != nil {
-		r.writeEnv.Each(func(name string, vr expand.Variable) bool {
-			r.Vars[name] = vr
-			return true
-		})
+		maps.Insert(r.Vars, r.writeEnv.Each)
 	}
 	return r.err
 }

interp/vars.go

@@ -76,7 +76,7 @@ func (o *overlayEnviron) Each(f func(name string, vr expand.Variable) bool) {
 
 func execEnv(env expand.Environ) []string {
 	list := make([]string, 0, 64)
-	env.Each(func(name string, vr expand.Variable) bool {
+	for name, vr := range env.Each {
 		if !vr.IsSet() {
 			// If a variable is set globally but unset in the
 			// runner, we need to ensure it's not part of the final
@@ -92,8 +92,7 @@ func execEnv(env expand.Environ) []string {
 		if vr.Exported && vr.Kind == expand.String {
 			list = append(list, name+"="+vr.String())
 		}
-		return true
-	})
+	}
 	return list
 }