*: 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.go

@@ -0,0 +1,48 @@
+package mtree
+
+import (
+	"bufio"
+	"io"
+)
+
+const posixPolynomial uint32 = 0x04C11DB7
+
+func cksum(r io.Reader) (uint32, int, error) {
+	in := bufio.NewReader(r)
+	count := 0
+	var sum uint32 = 0
+	f := func(b byte) {
+		for i := 7; i >= 0; i-- {
+			msb := sum & (1 << 31)
+			sum = sum << 1
+			if msb != 0 {
+				sum = sum ^ posixPolynomial
+			}
+		}
+		sum ^= uint32(b)
+	}
+
+	for done := false; !done; {
+		switch b, err := in.ReadByte(); err {
+		case io.EOF:
+			done = true
+		case nil:
+			f(b)
+			count++
+		default:
+			return ^sum, count, err
+		}
+	}
+	for m := count; ; {
+		f(byte(m) & 0xff)
+		m = m >> 8
+		if m == 0 {
+			break
+		}
+	}
+	f(0)
+	f(0)
+	f(0)
+	f(0)
+	return ^sum, count, nil
+}