@kha I have added support for opaque constants to the old C++ frontend,
and made sure the new frontend can still parse `library/init/core.lean`.
The kernel should enforce that opaque constants are really opaque, and
the following example should fail
```
constant x : nat := 0
theorem foo : x = 0 := rfl
```
If it doesn't, it is a bug.
Here are some remaining issues:
1- `environment.mk_empty` is currently an axiom because we cannot create
an inhabitant of an opaque type. A possible solution is to use
`option environment` instead of `environment`.
2- There is no support for opaque constants in the new
frontend. However, I modified it to handle axioms, and fixed the literal
values with decl_cmd_kind. I tried to mark some of my changes with
comments, but it is probably much easier for you to just check the
commit change list.
3- I did not add any support for automatically constructing `e`
at `constant x : t := e`. I think we can do this later
after we replace the old frontend with the new one. BTW, it took only a
few minutes to provide the inhabitants manually.
