Lexical variables aren't shown in the debugger for interpreted code

Regression introduced in !346 (merged)

Example:

> (defun f (x) (1+ x))

F
> (f nil)


Condition of type: SIMPLE-TYPE-ERROR
In function 1+, the value of the only argument is
  NIL
which is not of the expected type NUMBER
Available restarts:

1. (RESTART-TOPLEVEL) Go back to Top-Level REPL.

Broken at 1+. In: #<process TOP-LEVEL 0x7f557983af00>.
>> :d

Broken at F. In: #<process TOP-LEVEL 0x7f557983af00>.

>> :v

Local variables: (none) ; Should show X here but doesn't!
Special variables: (none)

We need to change decode-ihs-env in top.lsp to deal with the new representation for lexicals.