Xml escape

[bbbgan]

Support escape and unescape.
During serialization, the marco XML_ESCAPE_UNICODE control the attribute delimiter, and the macro XML_ESCAPE_UNICODE control if escape the unicode.

example:

  std::string str = R"(
  <text_t description="&quot;&lt;'&#x5c0f;&#24378;'&gt;&quot;">
    <ID ID'msg='{"msg&apos;reply": "it&apos;s ok"}'>&amp;&lt;&gt;</ID>
    <DisplayName>&#x5c0f;&#24378;</DisplayName>
  </text_t>
  )";
  using text_attr_t =
      iguana::xml_attr_t<text_t, std::map<std::string_view, std::string>>;
  auto validator = [](const text_attr_t& text) {
    auto v = text.value();
    auto attr = text.attr();
    assert(attr["description"] == R"("<'小强'>")");
    assert(v.ID.value() == R"(&<>)");
    assert(v.ID.attr()["ID'msg"] == R"({"msg'reply": "it's ok"})");
    assert(v.DisplayName == "小强");
  };
  text_attr_t text;
  iguana::from_xml(text, str);
  validator(text);
  std::string ss;
  iguana::to_xml<true>(text, ss);
  std::cout << ss << std::endl;

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 47.05882% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 44.98%. Comparing base (fb6e6d5) to head (639fa70).

❗ Current head 639fa70 differs from pull request most recent head 4cc6cfb. Consider uploading reports for the commit 4cc6cfb to get more accurate results

Files	Patch %	Lines
example/xml_example.cpp	0.00%	16 Missing ⚠️
iguana/util.hpp	0.00%	16 Missing ⚠️
iguana/xml_util.hpp	0.00%	3 Missing ⚠️
iguana/xml_reader.hpp	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
- Coverage   45.02%   44.98%   -0.04%     
==========================================
  Files          46       46              
  Lines        5628     5644      +16     
==========================================
+ Hits         2534     2539       +5     
- Misses       3094     3105      +11     

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[qicosmos]

LGTM