Fix: char-code for *whitespaces* characters

[kilianmh]

Use char-code for *whitespaces* characters.
This should prevent future problems with some
implementations that do not support specific
characters in the reader.

This should work regardless of implementation.
Tested on: sbcl, ccl, ecl, abcl, cmu, allegro.

From now on we should add characters
only with char-code to avoid errors? @vindarel

[vindarel]

oooh this looks wise, but we are loosing a user-facing QoL.

We can easily see what's in the whitespaces variable:

str:*whitespaces*
(#\Backspace #\Tab #\Newline #\Newline #\Vt #\Page #\Return #\  #\Rubout
 #\Next-Line #\NO-BREAK_SPACE)

not so with charcodes.

I really like cl-str to be easily inspectable, clear, etc. Maybe there's a better way. Or we maintainers have to work harder and add some tests / a CI for the future.

[kilianmh]

not so with charcodes.

Is it? I get the same result with this PR when putting str:*whitespaces* in the repl.

Also there are comments after every character-code with the respective character.