util: move ArrayReverse into package of its own

Leave just uint160 and uint256 types in util.

pkg/util/slice/array.go

@@ -0,0 +1,14 @@
+/*
+Package slice contains byte slice helpers.
+*/
+package slice
+
+// CopyReverse returns a new byte slice containing reversed version of the
+// original.
+func CopyReverse(b []byte) []byte {
+	dest := make([]byte, len(b))
+	for i, j := 0, len(b)-1; i <= j; i, j = i+1, j-1 {
+		dest[i], dest[j] = b[j], b[i]
+	}
+	return dest
+}