rand: add explicit Int31n to avoid 64-bit divide on 32-bit machines

use Int31n in Intn when possible.

Fixes #390.

(using 8g)
Intn1000      50000000         38 ns/op
Int31n1000    50000000         39 ns/op
Int63n1000    20000000        114 ns/op

R=r
CC=golang-dev, skybrian
https://golang.org/cl/180054

src/pkg/rand/rand_test.go

@@ -327,3 +327,24 @@ func BenchmarkInt63Unthreadsafe(b *testing.B) {
 		r.Int63()
 	}
 }
+
+func BenchmarkIntn1000(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Intn(1000)
+	}
+}
+
+func BenchmarkInt63n1000(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Int63n(1000)
+	}
+}
+
+func BenchmarkInt31n1000(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Int31n(1000)
+	}
+}