Improve Box in EnvLang

[myreen]

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.