We'd like to cleanup the `proof_end` type so we can have a smaller
path in proof save. Note that the construction:
```
Goal Type.
⋮
Save id.
```
has to be handled by the STM in the same path as Defined (but with an
opaque flag), as `Save id` will alter the environment and cannot be
processed in parallel.
We thus try to simply such paths a bit, as complexity of `lemmas.ml`
seems like an issue these days. The form `Save Theorem id` doesn't
really seem used, and moreover we should really add a type of "Goal",
and unify syntax.
It is often the case that beginners try `Goal addnC n : n + 0 = n."
etc...