Add parseFlakeRef and flakeRefToString builtins

[aakropotkin]

Motivation

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.

Context

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.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• documentation in the internal API docs
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

Add 👍 to pull requests you find important.

[aakropotkin]

@edolstra bump ❤️

[Ericson2314]

With those docs changes
(we don't really worry about line length in the multiline strings, but rather try to use 1 sentance per line.)
and the release notes conflicts fixed (because 2.17 was just cut) this is good!

[Ericson2314]

note the release notes still have a conflict, because 2.17 was just released.

[aakropotkin]

note the release notes still have a conflict, because 2.17 was just released.

Got that conflict resolve and merged the provided doc changes ( thanks for tweaking those ).

[thufschmitt]

Coming a bit late, but from the 2023-07-24 Nix maintainers meeting:

Approved on the principle, assigned to @Ericson2314

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-07-24-nix-team-meeting-minutes-74/31116/1