Release candidate for 2.0.0

Small fixes over beta5

This new beta brings many usability improvements and fixes. Most notably:

• A new, clearer and more expressive format for depexts:. Opam can now directly extract them for the current system (opam-depext is still required to talk to the host package manager, although hooks are even possible to make that automatic)
• Optimised use of the built-in solver, glpk is now also built in, hence not required to compile opam
• Support for "locked" opam files and reproduceable build environments with the new pin-depends: field
• A fix for a bug that appeared with OCaml 4.05.0, where installed executables where not found (requiring extra eval $(opam env))
• Installed opam plugins can now be used even if the switch they were installed from is not currently selected

There is much more, please see the changelog for details.

NOTE: due to format changes, be careful when using the official 2.0 repository (at https://opam.ocaml.org/2.0): once it is switched to the new format, older betas will complain (depexts issues or post flags). Until then, this release will already work fine except for some possible quirks with depexts.

This beta brings many small fixes, some polish to the CLI, and most importantly a built-in solver, so it isn't needed to care about having aspcud installed anymore.

More detailed log:

• Building with OCaml < 4.02.3 is no longer supported
• Support compilation with a built-in mccs solver (removing run-time dependency to aspcud). Integrated half-working "heuristics" dropped.
• Remove jsonm and transitive uutf dependency.
• Switch build systems to jbuilder, including lib-ext support
• Allow repeated and or'd arguments to the list command
• Many code and build system portability fixes (Windows, BSD)
• Add switch export --full to include package definitions
• Add announce: and stamp: fields to repositories (repo files)
• Add a global cache of git objects (greatly speeding up multiple cloning)
• Allow opam pin URL without a package name
• Many error handling, messages and corner case fixes
• Updated the versions of dependencies
• Removed the unused features: field, and proposal for a new syntax
• More informative exit codes, and documentation thereof

This release brings a few speed and stability improvements, replaces the opam build command with opam install <dir> (see this announcement for details), makes the interface for handling packages' test and doc dependencies and instruction clearer, and extends the CLI in a few areas.

This bug-fix release fixes some small issues 1.2.1 had, and brings a couple improvements:

Fixes:

• The state-cache was broken on OSX, which could cause longer startup times
• opam config report didn't report the external solver correctly
• --dry-run --verbose didn't give the expected output
• Providing a simple path to an aspcud executable as external solver without expliciting the arguments works again
• Fixed a fd leak on solver calls
• opam list now returns 0 when no packages match but no pattern was supplied

Improvements:

• Conflict messages now report the original version constraints without translation, they are more concise in some cases
• Some new opam lint checks, opam lint now numbers its warnings and may provide script-friendly output
• Feature to automatically install plugins, e.g. opam depext will prompt to install depext if available and not already installed
• Priority to newer versions even when the latest can't be installed (with a recent solver only. Before, all non-latest versions were equivalent to the solver)
• Added opam list --resolve to list a consistent installation scenario
• Be cool by default on errors in opam files, for easier format updates without shouting at end-users

As usual, see the more detailed changelog for details.