Support for defining intermediary definitions in-between fields of an inductive constructor

Following a discussion in the F* meeting, it would be nice to be able to do something like:

```
type klass =
| Klass:
length:nat -> (
let t = s:seq nat { S.length s = length } in
lemma: (t -> ...) ->
klass)
```

I think Santiago and Guido both expressed an interest in having this feature.

Start by reviewing the proposed inductive-constructor syntax and the referenced F* meeting discussion. Identify the parser, elaboration, and type-checking entry points for constructor fields; done means intermediary let and lemma definitions are accepted between fields while the example typechecks.

Written by the indexing model from the issue text.