expand: skip brace expansion when not used

Makes expand.Fields noticeably faster, since Braces allocates quite a
bit.

While at it, make interp's execEnv assume that most environments are
larger. For example, on my fairly minimal laptop setup, I already have
about 45 environment variables.

name   old time/op    new time/op    delta
Run-8    1.52ms ± 1%    1.50ms ± 1%  -1.28%  (p=0.002 n=6+6)

name   old alloc/op   new alloc/op   delta
Run-8     185kB ± 0%     182kB ± 0%  -1.76%  (p=0.004 n=6+5)

name   old allocs/op  new allocs/op  delta
Run-8     1.14k ± 0%     1.05k ± 0%  -7.55%  (p=0.002 n=6+6)

expand/expand.go

@@ -314,8 +314,11 @@ func Fields(cfg *Config, words ...*syntax.Word) ([]string, error) {
 	fields := make([]string, 0, len(words))
 	dir := cfg.envGet("PWD")
 	for _, word := range words {
-		expWord, _ := syntax.SplitBraces(word)
-		for _, word2 := range Braces(expWord) {
+		expBraces := []*syntax.Word{word}
+		if expWord, any := syntax.SplitBraces(word); any {
+			expBraces = Braces(expWord)
+		}
+		for _, word2 := range expBraces {
 			wfields, err := cfg.wordFields(word2.Parts)
 			if err != nil {
 				return nil, err

interp/vars.go

@@ -39,7 +39,7 @@ func (o overlayEnviron) Each(f func(name string, vr expand.Variable) bool) {
 }
 
 func execEnv(env expand.Environ) []string {
-	list := make([]string, 0, 32)
+	list := make([]string, 0, 64)
 	env.Each(func(name string, vr expand.Variable) bool {
 		if vr.Exported {
 			list = append(list, name+"="+vr.String())