Replace classOf with a lookup table.

Improve performance of byte classification using a static lookup table.  The
improvement over a function is not as great as the gain from using a function
over a map, but it's enough to be worthwhile.

```
BENCHMARK                      BEFORE    AFTER    SPEEDUP (%)
BenchmarkSplit/len_1-12        570       563      1.2
BenchmarkSplit/len_4-12        612       600      2.0
BenchmarkSplit/len_16-12       714       704      1.4
BenchmarkSplit/len_64-12       1119      1082     3.3
BenchmarkSplit/len_256-12      2965      2799     5.6
BenchmarkSplit/len_1024-12     10216     8928     12.6
BenchmarkSplit/len_4096-12     39333     33102    15.8
BenchmarkSplit/len_16384-12    158126    130540   17.4
BenchmarkSplit/len_65536-12    669554    566011   15.5
BenchmarkSplit/len_100739-12   1039795   884622   14.9
```

1 files changed, 10 insertions, 18 deletions

diff --git a/shell.go b/shell.go
index b5a457d..e4f8650 100644
--- a/shell.go
+++ b/shell.go
@@ -52,12 +52,12 @@ const (
 type class int
 
 const (
-	clBreak class = iota
+	clOther class = iota
+	clBreak
 	clNewline
 	clQuote
 	clSingle
 	clDouble
-	clOther
 )
 
 type action int
@@ -140,21 +140,13 @@ var update = [...][]struct {
 	},
 }
 
-func classOf(b byte) class {
-	switch b {
-	case ' ', '\t':
-		return clBreak
-	case '\n':
-		return clNewline
-	case '\\':
-		return clQuote
-	case '\'':
-		return clSingle
-	case '"':
-		return clDouble
-	default:
-		return clOther
-	}
+var classOf = [256]class{
+	' ':  clBreak,
+	'\t': clBreak,
+	'\n': clNewline,
+	'\\': clQuote,
+	'\'': clSingle,
+	'"':  clDouble,
 }
 
 // A Scanner partitions input from a reader into tokens divided on space, tab,
@@ -190,7 +182,7 @@ func (s *Scanner) Next() bool {
 		} else if err != nil {
 			return false
 		}
-		next := update[s.st][classOf(c)]
+		next := update[s.st][classOf[c]]
 		s.st = next.state
 		switch next.action {
 		case push: