feat(Profpatsch): dump netencode spec & parser

The netencode standard, a no-nonsense extension of netstrings for
structured data.

Includes a nix generator module and a rust parsing library.

Imported from
e409df3861/pkgs/profpatsch/netencode

Original license GPLv3, but I’m the sole author, so I transfer it to
whatever license depot uses.

Change-Id: I4f6fa97120a0fd861eeef35085a3dd642ab7c407
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2319
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/netencode/netencode.nix

@@ -0,0 +1,49 @@
+let
+
+  netstring = tag: suffix: s:
+    "${tag}${toString (builtins.stringLength s)}:${s}${suffix}";
+
+  unit = "u,";
+
+  n1 = b: if b then "n1:1," else "n1:0,";
+
+  n = i: n: netstring "n${toString i}" "," (toString n);
+  i = i: n: netstring "i${toString i}" "," (toString n);
+
+  n3 = n 3;
+  n6 = n 6;
+  n7 = n 7;
+
+  i3 = i 3;
+  i6 = i 6;
+  i7 = i 7;
+
+  text = netstring "t" ",";
+  binary = netstring "b" ",";
+
+  tag = key: val: netstring "<" "|" key + val;
+
+  concatStrings = builtins.concatStringsSep "";
+
+  record = lokv: netstring "{" "}"
+    (concatStrings (map (kv: tag kv.key kv.val) lokv));
+
+  list = l: netstring "[" "]" (concatStrings l);
+
+in {
+  inherit
+    unit
+    n1
+    n3
+    n6
+    n7
+    i3
+    i6
+    i7
+    text
+    binary
+    tag
+    record
+    list
+    ;
+}