The experimental function signature is only available when
`GIT_EXPERIMENTAL_SHA256` is enabled.

libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.

Linux has a /usr/include/features.h, which gets confusing; update this
to `git2_features.h` and move it into the `util` directory.

The git_odb_hash helper functions should not assume SHA1, and instead
should be given the oid type that they're producing.

Avoid too much flashing on the console with updates about throughput.
Only update throughput once a second.

Provide a mechanism to add a signal handler for Unix or Win32.

Provide a class that will display progress information to the console.
Initially, it contains callbacks for fetch progress and checkout
progress.