*: entry linking and keyword filter

Setting up sibling and parent relationships for entries, so they can be
easier to walk.

Also, making "keyword=value" easier to parse. This helps filtering.

Both of these ready us for checking/validating a hierarchy.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

walk_test.go

@@ -2,6 +2,7 @@ package mtree
 
 import (
 	"io/ioutil"
+	"log"
 	"testing"
 )
 
@@ -11,6 +12,8 @@ func TestWalk(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	log.Fatalf("%#v", dh)
+
 	fh, err := ioutil.TempFile("", "walk.")
 	if err != nil {
 		t.Fatal(err)