feat(sterni/nix/url): implement urldecoding

We use builtins.split directly as it should be a bit more efficient as
lib.splitStrings. Also its returning of a list for every regex match is
useful to update the state while parsing the tokens:

* The tokens are obtained by splitting the string at every '%'
* Everytime we see a boundary (that is a list in the returned
  list of builtins.split), we know that the first two chars of
  the next string are a percent encoded character.

One implementation flaw is that it will currently crash if it encounters
mal-formed URLs (since int.fromHex chrashes if it encounters any non
hex digit characters) and accepts some malformed urlencoding like
"foo %A".

Change-Id: I90d08d7a71b16b4f4a4879214abd7aeff46c20c8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2744
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/nix/url/tests/default.nix

@@ -16,6 +16,10 @@ let
     assertEq "encode ${builtins.toJSON left} == ${builtins.toJSON right}"
       (url.encode args left) right;
 
+  checkDecoding = { left, right }:
+  assertEq "${builtins.toJSON left} == decode ${builtins.toJSON right}"
+    (url.decode left) right;
+
   unreserved = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.-_~";
 
   encodeExpected = [
@@ -31,6 +35,9 @@ let
   testEncode = it "checks url.encode"
     (builtins.map (checkEncoding {}) encodeExpected);
 
+  testDecode = it "checks url.decode"
+    (builtins.map checkDecoding encodeExpected);
+
   testLeaveReserved = it "checks that leaveReserved is like id for valid URLs"
     (builtins.map (x: checkEncoding { leaveReserved = true; } { left = x; right = x; }) [
       "ftp://ftp.is.co.za/rfc/rfc1808.txt"