CHANGES (p.0install-solver.2.18.doc.CHANGES)

Solver:

Add 0install-solver.opam. This makes the solver into a separate opam package.
Improve display of implementations in diagnostics. Instead of sha1=3ce644dc725f1d21cfcf02562c76f375944b266a (1), show: v1 (sha1=3ce644dc725f...).
Only report restrictions that affected the result. If a restriction didn't remove any candidates (either because it matched all of them or because another restriction already removed them) then don't bother reporting it. Also, don't bother reporting a restriction that only removed candidates worse than the selected one (if any). For user-provided restrictions, filter the rejects to show only the version the user asked for (unless that would remove all of them). Since we only show the first 5 rejects, this would often mean that the interesting candidates weren't even shown.
Expose more of the solver diagnostics API. This allows users to format the results in other ways (for example, as a list in a GUI). Also, use formatting boxes instead of manual indentation.
Add conflict_class. This isn't used by 0install, but the new opam backend that uses the 0install solver needs it.

Build:

Update minimum OCaml version to 4.08. Remove our custom Option module and use the new stdlib one instead. Use the new List.filter_map from the stdlib instead of our one. Also, rename our first_match to find_map, to match the name in 4.10.
Depend on ounit2 for unit-tests. The old oUnit is just a transition package that depends on ounit2 now.

Update to GTK 3, because Debian is removing GTK 2 support now. Note that the systray icon no longer blinks to indicate that action is required, as GTK removed this feature. This also updates the static Docker images to use Ubuntu 16.04, since 14.04's version of GTK 3 is too old.
Upgrade to dune 2.1.

Don't try to update distribution caches in --dry-run mode. If we needed to update a cache, the operation would fail with e.g. Bug: 'with_open_out' called in --dry-run mode.

Mark obus as required (except on Windows and macOS). This is more explicit than just asking users to install it where it makes sense, and also means that the CI will test it.
Split GTK UI into a separate opam package. This makes it easy to install it (using opam install 0install-gtk), and means that the CI will test it.
Update opam metadata to match opam-repository.

Rename "ocaml" directory to "src". There are no other languages here now.
Make the solver return a SOLVER_RESULT. The old API was a bit confusing. The user of the solver had to wrap the solver's return to provide a SOLVER_RESULT for the diagnostics. Now, the user-required bits are in SOLVER_INPUT and the solver itself provides the rest.
Split the solver into its own library. Since the refactoring in 2014 (see Simplifying the Solver With Functors), the solver isn't really tied to 0install at all, and could be useful for other package managers.
Generalise the solver's machine groups system. This removes the one remaining dependency from the solver to the rest of 0install.
Rename option functions in solver to match OCaml 4.08.
Split Feed_metadata and Feed_import out into their own modules.
Rename Feed.feed to Feed.t and make the type abstract.
Skip rpm2cpio unit test if cpio isn't available. It seems that recent Fedora images provide rpm2cpio, but not cpio.

Replace repo.roscidus.com feeds with apps.0install.net (Bastian Eicher).
Update fallback host-Python check for new URIs. http://repo.roscidus.com/python/python is now https://apps.0install.net/python/python.xml, etc.

Fix use of nested Lwt_main.run. This is now an error with Lwt 5.0.0.
Fix bad error message when running as root with invalid $HOME. If we're running as root then we check that $HOME is owned by root to avoid putting root-owned files in a user's home directory. However, if $HOME doesn't exist then the error Unix.ENOENT was unhelpful.
Fix bad solver error. The diagnostics system didn't consider dependencies of <command> elements when trying to explain why a solve failed. It could therefore report Reason for rejection unknown if a solve failed due to a constraint inside one.
When using --may-compile on a local selection, set local-path. We already gave the full path in local-path for other selections, and without this an exception is thrown if we try to print the resulting tree. e.g. No digests found for '.'

Move windows C code to its own library. This allows us to use the new (enabled_if ...) feature to enable it only on Windows, simplifying the dune files and avoiding the need for cppo.
Get the windows_api object via the system object, rather than making it a special case.
Remove windows-specific flags from utils.c. Looks like these were left over from when there was more code here.