Auto merge of #17520 - camlorn:master, r=asajeffrey

Convert node serialization to a purely iterative algorithm.

We maintain a stack of open element nodes with their children count, popping from the top of the stack and closing when the count reaches zero.

Contrary to my comment in #16696, this is a purely iterative algorithm.  I just wasn't feeling sufficiently clever with respect to finding a relatively clean way until later.

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