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.

RFC: SHA256 proof of concept

Users will need to be able to specify the object id type for the given
object database; add a new `git_odb_options` with that option.

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

`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.