odb: Expose git_odb_object_type() as OdbObject.Type()
odb: Expose git_odb_object_type() as OdbObject.Type()

It might be needed when one is writing `git cat-file --batch` equivalent
which has output format

    <sha1> SP <type> SP <size> LF
    <contents> LF

libgit2/git2go,pks-t/git2go,clns/git2go,ezwiebel/git2go,ezwiebel/git2go,clns/git2go,pks-t/git2go,ezwiebel/git2go,libgit2/git2go,clns/git2go,libgit2/git2go