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.

libgit2's current default oid type is SHA1, set a public macro for that.

`git_oid`s now have a type, and we require the oid type when creating
the object id from creation functions.

We require an enumeration to help us identify the different types of
object IDs.  Currently, we only support SHA1 but we will support SHA256
in the future.

Callers should not assume the layout of the oid structure; provide them
a macro that defines the null / zero sha1 object id.

In preparation for SHA256 support, `GIT_OID_RAWSZ` and `GIT_OID_HEXSZ`
need to indicate that they're the size of _SHA1_ OIDs.

Stop returning a void for functions, future-proofing them to allow them
to fail.