Scheme: Using brackets in addition to parentheses

[Werner LEMBERG]

As I've just discovered, brackets work the same as parentheses in
Scheme expressions:

```
#[set-global-staff-size 26]

{ c' }
```

Guile allows this by default; the read option `square-brackets` is
active for compatibility with R6RS.

Shall we use and advertise this?  Some user Scheme code could probably
be made more readable:

```
(let [(foo ...)
      (bar ...)]
  ...)

(cond
  [question1 expression1]   
  [question2 expression2]   
  [question3 expression3]   
  [else      expression4])
```


    Werner