ToString instance for Xml does not correctly escape

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While the XML parser correctly handles XML escape sequences, the stringifier does not emit them.

Zulip thread. This leads to some mangled markdown in doc-gen, which unknowingly tries to work around this bug.

Run the following:

import Lean

run_cmd do
  let .ok doc := Lean.Xml.parse "<b class=\"&quot;\">hello &lt; world </b>" | unreachable!
  Lean.logInfo (toString doc)

Expected behavior: The output is <b class=""">hello < world </b>

Actual behavior: The output is <b class=""">hello < world </b>, which is invalid XML

4.9.0-rc1
4.10.0-nightly-2024-06-09

[Additional information, configuration or data that might be necessary to reproduce the issue]

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leanprover / lean4