Users should provide us an array of object ids; we don't need a separate
type. And especially, we should not be mutating user-providing values.
Instead, use `git_oid *` in the shallow code.

Teach the smart transport more about oid types, instead of assuming SHA1.

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.

Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.