Simplify escapeHTML and uncomment it's benchmark

Simplify and optimize escapeHTML as per @Ambrevar's suggestion: lean on
the fact that we're dealing with bytes: declare a 256-element array with
most of it filled with zeros, except for the few slots that need
escaping. This avoids some conditionals in a tight loop.

Also, uncomment it's benchmark.

esc.go

@@ -5,32 +5,21 @@ import (
 	"io"
 )
 
-type escMap struct {
-	char byte
-	seq  []byte
-}
-
-var htmlEscaper = []escMap{
-	{'&', []byte("&")},
-	{'<', []byte("&lt;")},
-	{'>', []byte("&gt;")},
-	{'"', []byte("&quot;")},
+var htmlEscaper = [256][]byte{
+	'&': []byte("&amp;"),
+	'<': []byte("&lt;"),
+	'>': []byte("&gt;"),
+	'"': []byte("&quot;"),
 }
 
 func escapeHTML(w io.Writer, s []byte) {
 	var start, end int
-	var sEnd byte
 	for end < len(s) {
-		sEnd = s[end]
-		if sEnd == '&' || sEnd == '<' || sEnd == '>' || sEnd == '"' {
-			for i := 0; i < len(htmlEscaper); i++ {
-				if sEnd == htmlEscaper[i].char {
-					w.Write(s[start:end])
-					w.Write(htmlEscaper[i].seq)
-					start = end + 1
-					break
-				}
-			}
+		escSeq := htmlEscaper[s[end]]
+		if escSeq != nil {
+			w.Write(s[start:end])
+			w.Write(escSeq)
+			start = end + 1
 		}
 		end++
 	}

esc_test.go

@@ -23,7 +23,6 @@ func TestEsc(t *testing.T) {
 	}
 }
 
-/*
 func BenchmarkEscapeHTML(b *testing.B) {
 	tests := [][]byte{
 		[]byte(""),
@@ -33,7 +32,7 @@ func BenchmarkEscapeHTML(b *testing.B) {
 		[]byte("4 < 5."),
 		[]byte("6 > 5."),
 		[]byte("Here's a [link] [1] with an ampersand in the URL."),
-		[]byte("Here's a link with an amersand in the link text: [AT&T] [2]."),
+		[]byte("Here's a link with an ampersand in the link text: [AT&T] [2]."),
 		[]byte("Here's an inline [link](/script?foo=1&bar=2)."),
 		[]byte("Here's an inline [link](</script?foo=1&bar=2>)."),
 		[]byte("[1]: http://example.com/?foo=1&bar=2"),
@@ -47,4 +46,3 @@ func BenchmarkEscapeHTML(b *testing.B) {
 		}
 	}
 }
-*/