refactor(library/logic/prop): allow 'absurd' to generate Type
refactor(library/logic/if): mark basic theorem transparent
refactor(library/data/nat): declare lt and le asap using inductive definitions, and make key theorems transparent for definitional package

We also define key theorems that will be used to generate the
automatically generated a well-founded subterm relation for inductive
datatypes.
We also prove decidability and wf theorems asap.