This is much of the plumbing for the object database to support SHA256,
and for objects to be able to parse SHA256 versions of themselves.

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

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

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.

The `git2internal` target is actually the git library; call it such so
that IDE users have visibility into it.