go/printer, gofmt: fine tuning of line spacing

- no empty lines inside empty structs and interfaces
- top-level declarations are separated by a blank line if
  a) they are of different kind (e.g. const vs type); or
  b) there are documentation comments associated with a
     declaration (this is new)
- applied gofmt -w misc src

The actual changes are in go/printer/nodes.go:397-400 (empty structs/interfaces),
and go/printer/printer.go:307-309 (extra line break). The remaining
changes are cleanups w/o changing the existing functionality.

Fixes issue  2570.

R=rsc
CC=golang-dev
https://golang.org/cl/5493057

src/pkg/encoding/gob/codec_test.go

@@ -102,12 +102,15 @@ func TestIntCodec(t *testing.T) {
 
 // The result of encoding a true boolean with field number 7
 var boolResult = []byte{0x07, 0x01}
+
 // The result of encoding a number 17 with field number 7
 var signedResult = []byte{0x07, 2 * 17}
 var unsignedResult = []byte{0x07, 17}
 var floatResult = []byte{0x07, 0xFE, 0x31, 0x40}
+
 // The result of encoding a number 17+19i with field number 7
 var complexResult = []byte{0x07, 0xFE, 0x31, 0x40, 0xFE, 0x33, 0x40}
+
 // The result of encoding "hello" with field number 7
 var bytesResult = []byte{0x07, 0x05, 'h', 'e', 'l', 'l', 'o'}