Spin-off syscall-related file operations (#78)

* [file_utils] spin-off syscall-related file operations

1. Eliminate the potential circular dependency between
   persistent-data/block.h and persistent-data/file_utils.h,
   if the former one wants to include the latter.
2. Avoid namespace pollution by removing the "using namespace std"
   declaration in block.tcc.
3. Correct the header hierarchy: base/xml_utils.h now no longer
   depends on the higher-level persistent-data/file_utils.h

* [file_utils] support block files in get_file_length()

era/superblock.cc

@@ -3,10 +3,13 @@
 #include "persistent-data/checksum.h"
 #include "persistent-data/errors.h"
 
+#include <sstream>
+
 using namespace base;
 using namespace era;
 using namespace superblock_damage;
 using namespace persistent_data;
+using namespace std;
 
 //----------------------------------------------------------------