Add parseFlakeRef and flakeRefToString builtins (#8670)

Over the last year or so I've run into several use cases where I need to
parse and/or serialize URLs for use by `builtins.fetchTree` or
`builtins.getFlake`, largely in order to produce _lockfile-like_ files
for lang2nix frameworks or tools which use `nix` internally to drive
builds.

I've gone through the painstaking process of emulating
`nix::FlakeRef::fromAttrs` and `nix::parseFlakeRef` several times with
mixed success; but these are difficult to create and even harder to
maintain if I hope to stay aligned with changes to the real
parser/serializer.

I understand why adding new `builtins` isn't something we want to do
flagrantly. I'm recommending this addition simply because I keep
encountering use cases where I need to parse/serialize these URIs in
`nix` expressions, and I want a reliable solution.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: John Ericson <git@JohnEricson.me>

tests/lang/eval-okay-flake-ref-to-string.exp

@@ -0,0 +1 @@
+"github:NixOS/nixpkgs/23.05?dir=lib"

tests/lang/eval-okay-flake-ref-to-string.nix

@@ -0,0 +1,7 @@
+builtins.flakeRefToString {
+  type  = "github";
+  owner = "NixOS";
+  repo  = "nixpkgs";
+  ref   = "23.05";
+  dir   = "lib";
+}

tests/lang/eval-okay-parse-flake-ref.exp

@@ -0,0 +1 @@
+{ dir = "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type = "github"; }

tests/lang/eval-okay-parse-flake-ref.nix

@@ -0,0 +1 @@
+  builtins.parseFlakeRef "github:NixOS/nixpkgs/23.05?dir=lib"