This adds an example implementation that emulates git cat-file.
It is a convenient and relatively simple example of getting data
out of a repository.

Implementing this also revealed that there are a number of APIs
that are still not using const pointers to objects that really
ought to be.  The main cause of this is that `git_vector_bsearch`
may need to call `git_vector_sort` before doing the search, so a
const pointer to the vector is not allowed.  However, for tree
objects, with a little care, we can ensure that the vector of
tree entries is always sorted and allow lookups to take a const
pointer.  Also, the missing const in commit objects just looks
like an oversight.

Removed useless prototype and renamed object typecast functions
declaration macro.

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.

This unifies the object parse functions into one signature that
takes an odb_object.

This adds create and free callback to the git_objects_table so
that more of the creation and destruction of objects can be table
driven instead of using switch statements.  This also makes the
semantics of certain object creation functions consistent so that
we can make better use of function pointers.  This also fixes a
theoretical error case where an object allocation fails and we
end up storing NULL into the cache.

This uses the odb object accessors so we can change the internals
more easily...