Use hexadecimal, not decimal, for character references in all instances

All instances of numerical character references, also in examples,
should use hexadecimal notation.  Unicode 2.0 and later as well as
10646-1:2000 only refer to characters in hexadecimal, never decimal
(10646-1:1993 did, but was only confusing, and was removed).
(Decimal should still be allowed, but don't use that.)

So, instead of:
<!ENTITY nbsp   "&#160;" ><!-- no-break space = non-breaking space, U+00A0
ISOnum -->
<!ENTITY iexcl  "&#161;" ><!-- inverted exclamation mark, U+00A1 ISOnum -->

write:
<!ENTITY nbsp   "&#xA0;" ><!-- no-break space = non-breaking space, U+00A0
ISOnum -->
<!ENTITY iexcl  "&#xA1;" ><!-- inverted exclamation mark, U+00A1 ISOnum -->

Some alias names are useful; however, *NOT* the "Unicode 1.0" names,
which should be removed from the comments.


While on the subject: all of the named character entities should
be deprecated, they only make the source hard to read.
And numerical character referenced (hexadecimal please) should
only be used when the referenced character cannot be represented
in the actual (target or source) character encoding of the document.