Rename `digit-char-p' to `char-digit-p'.

Michael Sperber writes:
 > 
 > The name `digit-char-p' was moved to the core here:
 > 
 > 2015-02-25  Aidan Kehoe  <[email protected]>
 > 
 > 	* cl-extra.el (digit-char-p): Moved to data.c.
 > 	* cl-extra.el (digit-char): Moved to data.c.
 > 
 > 
 > Unfortunately, this breaks Gnus which defines the same name.  So:

Does it define it differently?  Since you say "break" I suppose it
must.

Either way, why is Gnus defining functions in the global namespace?

I don't really care about the name, but I think it's really obnoxious
to have *both* `digit-char-p' and `char-digit-p' defined.  Especially
if the semantics are different.