Convert deeply nested documents without recursion (#276)

[HardMax71]

Fixes GH-276. process_tag recursed for every nesting level, so an ordinary acyclic tree hit the default recursion limit at about 330 levels. Mail clients produce that shape easily, Outlook wraps every quoted reply in another div.

process_tag now walks the subtree with an explicit stack. The per-tag work moved into _open_tag (pick the children, build the parent context) and _close_tag (join, collapse newlines, run the convert function), unchanged otherwise. Traversal order is the same, so the output is identical and the existing suite passes untouched. bs4 did the same for tree output in 4.12.1 (bug 1471755).

Subclasses that override process_tag or process_element keep their per-child calls, so per-tag filters found in the wild still fire for nested tags (covered by a test). A cyclic tree (#256) would now spin instead of raising, so a tag already on the current path is skipped, same outcome as #274.

Checked on 3.8 with pytest, flake8, mypy and rst-lint as in CI. A 1000 level document converts in about 0.1 s.

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