*: loads of walking features

For the most part, all the keywords for a standard mtree spec now have a
function to produce the contents for a creator.

These are used in the "walk" function, and will be used next in the
"check" logic.

This is still a WIP, as the DirectoryHierarchy produced from the current
Walk() is not all-together a valid document.

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

cksum_test.go

@@ -0,0 +1,30 @@
+package mtree
+
+import (
+	"os"
+	"testing"
+)
+
+var (
+	checkFile        = "./testdata/source.mtree"
+	checkSum  uint32 = 1048442895
+	checkSize        = 9110
+)
+
+func TestCksum(t *testing.T) {
+	fh, err := os.Open(checkFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer fh.Close()
+	sum, i, err := cksum(fh)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if i != checkSize {
+		t.Errorf("%q: expected size %d, got %d", checkFile, checkSize, i)
+	}
+	if sum != checkSum {
+		t.Errorf("%q: expected sum %d, got %d", checkFile, checkSum, sum)
+	}
+}