[CM] s7 ((*repl* 'restore-repl)) breaks the repl

[Brad Christensen]

I've begun to explore the s7 repl via the manual. Testing a repl save and restore gave me a problem however:

```
$ s7i
s7: 6-Jul-2021
<1> (define (func n) 
      (+ n 1))
func
<2> (func 1)
2
<3> ((*repl* 'save-repl))
#<unspecified>
<4> (set! (*repl* 'top-level-let) (sublet (rootlet)))
(inlet)
<5> func
error: unbound variable func
<6> ((*repl* 'restore-repl))
func
<7> func
error: eval argument 2, func, is a function but should be a let (environment)
<8> (func 1)
error: eval argument 2, func, is a function but should be a let (environment)
<9> 42
error: eval argument 2, func, is a function but should be a let (environment)
<10> (*repl* 'top-level-let)
error: eval argument 2, func, is a function but should be a let (environment)
```

As an aside, another thing I noticed while looking at repl.scm was the `restore-repl` function does not take a filename argument, while the manual says it does.

Cheers, Brad