feat(structures): use num instead of nat
feat(structures): use num instead of nat

leanprover/tutorial,soonhokong/lean-tutorial,avigad/tutorial,Hodapp87/cs6460_project,Hodapp87/cs6460_project,cjmazey/fork_leanprover_tutorial,avigad/tutorial,soonhokong/lean-tutorial,soonhokong/lean-tutorial,fpvandoorn/tutorial,avigad/tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,Hodapp87/cs6460_project,fpvandoorn/tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial