remove "using namespace std" from headers

It's nasty, and actually breaks on Solaris, where if.h fails to
build due to:

  struct map *if_memmap;

src/cryptonote_core/tx_pool.cpp

@@ -51,6 +51,8 @@
 
 DISABLE_VS_WARNINGS(4244 4345 4503) //'boost::foreach_detail_::or_' : decorated name length exceeded, name was truncated
 
+using namespace crypto;
+
 namespace cryptonote
 {
   namespace