Improve Box in EnvLang

Currently, EnvLang's `Box` expression is:
```
exp = ... | Var vname | Box exp | ...
```
but it should arguably be:
```
exp = ... | Var vname | Box vname | ...
```
because one can always do `Let (SOME v) e (Box v)` where one would have done `Box e` before.

This issue is about three changes:
1. Making the change above to the expression syntax of EnvLang.
2. Allowing `Box` in `Letrec`, which currently only allows `Delay` and `Lam`.
3. Making sure that #37 replaces `Delay (Var v)` with `Box v` also in `Letrec`.

Start by locating the EnvLang expression definition and the Letrec handling for Delay and Lam. Inspect the related changes from #37, then update the Box syntax and Letrec cases so the requested Box form and Delay-to-Box replacement are covered; done means all three listed changes are implemented and validated by the existing tests.

Written by the indexing model from the issue text.