compress/flate: use seperate const block for exported constants

As rendered on https://tip.golang.org/pkg/compress/flate/, there is an
extra new-line because of the unexported constants in the same block.

<<<
const (
    NoCompression      = 0
    BestSpeed          = 1
    BestCompression    = 9
    DefaultCompression = -1
    HuffmanOnly        = -2 // Disables match search and only does Huffman entropy reduction.

)
>>>

Instead, seperate the exported compression level constants into its own
const block. This is both more readable and also fixes the issue.

Change-Id: I60b7966c83fb53356c02e4640d05f55a3bee35b7
Reviewed-on: https://go-review.googlesource.com/23557Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/compress/flate/deflate.go

@@ -16,9 +16,12 @@ const (
 	BestCompression    = 9
 	DefaultCompression = -1
 	HuffmanOnly        = -2 // Disables match search and only does Huffman entropy reduction.
-	logWindowSize      = 15
-	windowSize         = 1 << logWindowSize
-	windowMask         = windowSize - 1
+)
+
+const (
+	logWindowSize = 15
+	windowSize    = 1 << logWindowSize
+	windowMask    = windowSize - 1
 
 	// The LZ77 step produces a sequence of literal tokens and <length, offset>
 	// pair tokens. The offset is also known as distance. The underlying wire