feat(sterni/mn2html): reimplement mnote-html in Rust

Reimplement the MIME body extraction and HTML rewriting from mblog in
Rust so that shelling out to it becomes viable. The problem with
mnote-html is mainly that it – being written in CL – requires a ~300MB
executable and is a bit sluggish starting.

Change-Id: I5c1adc1a7ab5f3dde207f9a1f67ace685bd3f69f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13014
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: sterni <sternenseemann@systemli.org>

users/sterni/mn2html/README.md

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+# mn2html
+
+Convert mail notes authored e.g. by the iOS/macOS Notes application,
+into HTML suitable for standard browsers. Instead of full documents,
+mn2html emits HTML fragments that can easily be embedded into other
+documents or postprocessed using a templating engine.
+
+## History
+
+mn2html is a reimplementation mnote-html from //users/sterni/mblog.
+The reason for this was mainly avoiding the startup cost associated
+with Common Lisp programs, so the program would be suitable for
+shell scripting.
+
+## Tasks
+
+- [ ] Properly handle `text/plain` bodies (from e.g. notemap)
+- [ ] Add man page
+- [ ] Help screen
+- [ ] Improve error reporting