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/default.nix

@@ -0,0 +1,25 @@
+{ pkgs, lib, ... }:
+
+pkgs.rustPlatform.buildRustPackage rec {
+  pname = "mn2hmtl";
+  version = "canon";
+
+  src = lib.fileset.toSource {
+    root = ./.;
+    fileset = lib.fileset.unions [
+      ./Cargo.lock
+      ./Cargo.toml
+      ./mn2html.rs
+    ];
+  };
+
+  cargoLock.lockFile = ./Cargo.lock;
+
+  passthru.shell = pkgs.mkShell {
+    name = "${pname}-shell";
+    nativeBuildInputs = [
+      pkgs.buildPackages.cargo
+      pkgs.buildPackages.rustc
+    ];
+  };
+}