New constants needed

[edsel!eb@Navajo (Eric Benson)]

I suggest adding the following defined constants to the next edition of
Common Lisp:

STRING-CHAR-CODE-LIMIT
	The upper bound of character codes in string characters.
	(STRING-CHAR-P X) is equivalent to
	(AND (<= 0 (CHAR-CODE X))
	     (< (CHAR-CODE X) STRING-CHAR-CODE-LIMIT))

READTABLE-CHAR-CODE-LIMIT
	The upper bound of character codes for characters which may be
	readtable indices, i.e. legal first arguments to
	SET-SYNTAX-FROM-CHAR, SET-MACRO-CHARACTER, etc.

In many implementations CHAR-CODE-LIMIT is the same as
STRING-CHAR-CODE-LIMIT and READTABLE-CHAR-CODE-LIMIT, but not in all
implementations, for example Symbolics Common Lisp.