FEAT: Define a WHITESPACE charset

related to: metaeducation/rebol-issues#2189
mentioned: http://www.rebol.net/r3decide/0008.html

Using just the most basic _whitespace_ chars. Anyone may extended this charset if needed.
This one is compatible with Red language.

src/mezz/base-constants.r

@@ -32,21 +32,24 @@ sys: system/contexts/sys
 lib: system/contexts/lib
 
 ;-- Char constants:
-null:      #"^(NULL)"
+null:      #"^@"
 space:     #" "
 sp:        space
-backspace: #"^(BACK)"
+backspace: #"^H"
 bs:        backspace
 tab:       #"^-"
 newline:   #"^/"
 newpage:   #"^l"
 slash:     #"/"
 backslash: #"\"
-escape:    #"^(ESC)"
+escape:    #"^["
 cr:        #"^M"
 lf:        newline
 crlf:      "^M^J"
 
+;-- Bitset constants
+whitespace: #[bitset! #{0064000080}] ;= charset [#" " #"^-" #"^/" #"^M"]
+
 ;-- Function synonyms:
 q: :quit
 !: :not