feat(Profpatsch/writers): add rustSimple writers

A bunch of writer functions wrapping the `buildRustCrate`
functionality of nixpkgs. Can be used to write inline rust code, or
rust code read from files with `builtins.readFile`.

Change-Id: I9d74e9381b858b485925e4dc3fbb7fc392877c0a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2318
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/netstring/tests.nix

@@ -7,7 +7,8 @@ let
       ;
   };
 
-  python-netstring-test = imports.writers.python3 "python-netstring" {
+  python-netstring-test = imports.writers.python3 {
+    name = "python-netstring";
     libraries = p: [
       python-netstring
     ];