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nix: add nix-defined package and devel env (flake)

This adds a Nix Flake which defines:

- package recipies to build and run Mercurial (C, C+Rust variants),
- a meta package to run the test suite in an isolated sandbox,
- a development environment with some tools
  (notably Python and the Black formatter, pinned to match the CI).

Python is pinned to the recommended version.
The Rust toolchain is a fairly recent one provided by the Nixpkgs channel.
It is not yet pinned to the same version as the "reference" Debian package,
but this does not seem to cause any issue.

Example usage of local commands are provided in the `flake.nix` file.
Once merged, it should also be possible to pull and run directly from the
source repository directly with something like:
`nix run hg+https://foss.heptapod.net/mercurial/mercurial-devel?ref=TOPIC&dir=contrib/nix' -- version`

= Mercurial XXX =

== New Features ==

== Default Format Change ==

These changes affect newly created repositories (or new clones) done with
Mercurial XXX.

== New Experimental Features ==

== Bug Fixes ==

== Backwards Compatibility Changes ==

== Internal API Changes ==

== Miscellaneous ==