Fix compile errors due to name-hiding and an unused function argument.

These problems are pointed out by clang-tidy and fail compilation on msvc at /W4 /WX.

Signed-off-by: Daniela Engert <dani@ngrt.de>

test/prepare-test.cc

@@ -627,7 +627,7 @@ template <typename T> struct user_allocator {
     return new value_type[cnt];
   }
 
-  void deallocate(pointer p, size_type cnt) { delete[] p; }
+  void deallocate(pointer p, size_type) { delete[] p; }
 
   void construct(pointer p, const value_type& val) { new (p) value_type(val); }