feat(users/Profpatsch/{netencode,http-parse}): use HashMap

`U::Record` is required to be a hash map (later keys should be
ignored), so why not do the hash map immediately.

This surfaced a problem with read-http, because duplicate headers in
http are possible, but before they’d be silently ignored.
Now we merge them into a `U::List` in case, to be handled by
consumers of read-http.

Change-Id: Ifd594916f76e5acf9d08e705e0dec2c10a0081c9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2490
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/read-http.nix

@@ -2,7 +2,6 @@
 
 let
 
-  # reads a http request (stdin), and writes all headers to stdout, as netencoded dict
   read-http = depot.users.Profpatsch.writers.rustSimple {
     name = "read-http";
     dependencies = [