Add a new opam-0install-cudf package (@kit-ty-kate #11). This uses opam's CUDF API, allowing the solver to be used directly from within opam.
Dir_context.std_env now has some optional arguments, and also responds for opam-version (@talex5 #12). You will need to add an extra () argument to it to upgrade.
Evaluate a package's available expression in Dir_context (@talex5 #12). This isn't needed for Switch_context because the switch does it for us, but Dir_context could return packages with available: false.
Simplify the CONTEXT API (@talex5 #12). candidates now returns either Ok opam or Error pkg for each package. This is clearer than using an option type and avoids the need for a separate load function. It also makes it possible to filter packages based on the content of the opam file without having to load it twice. We also no longer bother loading the opam file for rejects (all we need is the name).