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.

The index's checksum is not an object ID, so we should not use the
`git_oid` type.  Use a byte array for checksum calculation and storage.

Deprecate the `git_indexer_hash` function.  Callers should use the new
`git_indexer_name` function which provides a unique packfile name.