Cleanup: put middleware helper functions in pkgs (#245)

Move all (almost all) Go files in middleware into their
own packages. This makes for better naming and discoverability.

Lot of changes elsewhere to make this change.

The middleware.State was renamed to request.Request which is better,
but still does not cover all use-cases. It was also moved out middleware
because it is used by `dnsserver` as well.

A pkg/dnsutil packages was added for shared, handy, dns util functions.

All normalize functions are now put in normalize.go

middleware/file/tree/less.go

@@ -0,0 +1,59 @@
+package tree
+
+import (
+	"bytes"
+
+	"github.com/miekg/dns"
+)
+
+// less returns <0 when a is less than b, 0 when they are equal and
+// >0 when a is larger than b.
+// The function orders names in DNSSEC canonical order: RFC 4034s section-6.1
+//
+// See http://bert-hubert.blogspot.co.uk/2015/10/how-to-do-fast-canonical-ordering-of.html
+// for a blog article on this implementation.
+//
+// The values of a and b are *not* lowercased before the comparison!
+func less(a, b string) int {
+	i := 1
+	aj := len(a)
+	bj := len(b)
+	for {
+		ai, oka := dns.PrevLabel(a, i)
+		bi, okb := dns.PrevLabel(b, i)
+		if oka && okb {
+			return 0
+		}
+		// sadly this []byte will allocate... TODO(miek): check if this is needed
+		// for a name, otherwise compare the strings.
+		ab := []byte(a[ai:aj])
+		bb := []byte(b[bi:bj])
+		doDDD(ab)
+		doDDD(bb)
+
+		res := bytes.Compare(ab, bb)
+		if res != 0 {
+			return res
+		}
+
+		i++
+		aj, bj = ai, bi
+	}
+	return 0
+}
+
+func doDDD(b []byte) {
+	lb := len(b)
+	for i := 0; i < lb; i++ {
+		if i+3 < lb && b[i] == '\\' && isDigit(b[i+1]) && isDigit(b[i+2]) && isDigit(b[i+3]) {
+			b[i] = dddToByte(b[i:])
+			for j := i + 1; j < lb-3; j++ {
+				b[j] = b[j+3]
+			}
+			lb -= 3
+		}
+	}
+}
+
+func isDigit(b byte) bool     { return b >= '0' && b <= '9' }
+func dddToByte(s []byte) byte { return (s[1]-'0')*100 + (s[2]-'0')*10 + (s[3] - '0') }