Read-time conditionals

It would be nice to have read-time conditionals acknowledged in coalton-mode. I'm assuming/hoping they will be supported in `.coal` files.

```
#+sbcl
(coalton-toplevel
  (define (myfunc x)
    (sbcl-specific-function x)))
#-sbcl
(coalton-toplevel
  (define (myfunc x)
    (general-function x))
    
```
The above should "comment out" whichever code is inapplicable.