regexp: add support for matching text read from things that implement

ReadRune.  (If you have a Reader but not a RuneReader, use bufio.)

The matching code is a few percent slower but significantly cleaner.

R=rsc
CC=golang-dev
https://golang.org/cl/4125046

src/pkg/regexp/find_test.go

@@ -6,6 +6,7 @@ package regexp
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 )
 
@@ -191,6 +192,12 @@ func TestFindStringIndex(t *testing.T) {
 	}
 }
 
+func TestFindReaderIndex(t *testing.T) {
+	for _, test := range findTests {
+		testFindIndex(&test, MustCompile(test.pat).FindReaderIndex(strings.NewReader(test.text)), t)
+	}
+}
+
 // Now come the simple All cases.
 
 func TestFindAll(t *testing.T) {
@@ -387,6 +394,12 @@ func TestFindStringSubmatchIndex(t *testing.T) {
 	}
 }
 
+func TestFindReaderSubmatchIndex(t *testing.T) {
+	for _, test := range findTests {
+		testFindSubmatchIndex(&test, MustCompile(test.pat).FindReaderSubmatchIndex(strings.NewReader(test.text)), t)
+	}
+}
+
 // Now come the monster AllSubmatch cases.
 
 func TestFindAllSubmatch(t *testing.T) {