Fix lifetime and copy issues with NodeDfsIterable

When running `node_traversal_black` with ASan and UBSan, there were two
distinct issues being reported. UBSan was complaining that we were
assigning an invalid value to a Boolean. This happened because
`d_initialized` in `NodeDfsIterator` was uninitialized and the default
copy constructor tried to copy it. This commit removes that data member.
ASan was complainig that `NodeDfsIterator::begin()` was trying to access
a skip function that had been freed. In particular, this happened when
`NodeDfsIterable` was used in a range-based for loop like so:

```
for (auto n : NodeDfsIterable(n).inPostorder())
{
  ...
}
```

The problem here is that the lifetime of a temporary _within_ the range
expression is not extended (the lifetime of the result of the range
expression is extended, however) [0]. This is a problem because
`NodeDfsIterable(n)` is a temporary and `inPostorder()` returns a
reference to that temporary. It makes sense that the compiler cannot
possibly know that the reference from `inPostorder()` corresponds to
`NodeDfsIterable(n)`, so it cannot extend its lifetime. See [1] for more
details on the problem with chained functions.

This commit fixes the issue by replacing the fluent interface of
`NodeDfsIterable` by a constructor with default arguments. Additionally,
it introduces an enum to represent the visit order to avoid having a
nondescript Boolean argument.

[0] https://en.cppreference.com/w/cpp/language/range-for#Temporary_range_expression
[1] http://cpptruths.blogspot.com/2018/10/chained-functions-break-reference.html?m=1

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