Users may want to validate raw object content; provide them a function
to do so.

Introduce `git_object_size_t`, an unsigned type that we can use for the
maximum size of git objects.

In #5118 we remove the double-underscore to make it a normally-named public
function. However, this is not an interesting function outside of the library
and it takes up a name for something that could be more useful.

Remove the single-underscore version as we have not done any releases with it.

We don't use double-underscores in the public API.

git_object_t is the future; update the public API to use it.  This will
also ensure that we can build our tests which make use of the old API
without modification (and without compiler warnings).

There are some combination of objects and target types which we know
cannot be fulfilled. Return EINVALIDSPEC for those to signify that there
is a mismatch in the user-provided data and what the object model is
capable of satisfying.

If we start at a tag and in the course of peeling find out that we
cannot reach a particular type, we return EPEEL.

Add a tag example