* Store attribute sets as a vector instead of a map (i.e. a red-black

tree).  This saves a lot of memory.  The vector should be sorted so
  that names can be looked up using binary search, but this is not the
  case yet.  (Surprisingly, looking up attributes using linear search
  doesn't have a big impact on performance.)

  Memory consumption for

    $ nix-instantiate /etc/nixos/nixos/tests -A bittorrent.test --readonly-mode

  on x86_64-linux with GC enabled is now 185 MiB (compared to 946
  MiB on the trunk).

src/libexpr/symbol-table.hh

@@ -28,6 +28,8 @@ private:
     friend class SymbolTable;
 
 public:
+    Symbol() : s(0) { };
+    
     bool operator == (const Symbol & s2) const
     {
         return s == s2.s;