Remove "using namespace std" from compiler header files and fix the fallout.

libmisc/StringHeap.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2002-2010 Stephen Williams ([email protected])
+ * Copyright (c) 2002-2021 Stephen Williams ([email protected])
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -29,6 +29,8 @@ static char **string_pool = NULL;
 static unsigned string_pool_count = 0;
 #endif
 
+using namespace std;
+
 static const unsigned DEFAULT_CELL_SIZE = 0x10000;