This fixes warning C5038 in Visual Studio 2017, which is caused by out-of-order initialization in the constructors of `heap_base`.
```
C:\dev\boost_1_70_0\boost/heap/detail/stable_heap.hpp(186):
warning C5038: data member 'boost::heap::detail::heap_base<T,std::function<bool (ttk::SimplexId,ttk::SimplexId)>,true,unsigned __int64,false>::cmp_' will be initialized after base class 'boost::heap::detail::size_holder<true,size_t>' with [ T=ttk::SimplexId ]
```
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