library(charsio): add to_upper and to_lower

[aarroyoc]

It adds uppercase and lowercase transformations based on #748

?- use_module(library(charsio)).
   true.
?- char_type(x, Type).
   Type = alnum
;  Type = alpha
;  Type = alphabetic
;  Type = alphanumeric
;  Type = ascii
;  Type = ascii_graphic
;  Type = lower
;  Type = octet
;  Type = prolog
;  Type = symbolic_hexadecimal
;  Type = to_lower("x")
;  Type = to_upper("X")
;  false.
?- char_type('ß', Type).
   Type = alnum
;  Type = alpha
;  Type = alphabetic
;  Type = alphanumeric
;  Type = lower
;  Type = octet
;  Type = prolog
;  Type = to_lower("ß")
;  Type = to_upper("SS")
;  false.
?- char_type('X', Type).
   Type = alnum
;  Type = alpha
;  Type = alphabetic
;  Type = alphanumeric
;  Type = ascii
;  Type = ascii_graphic
;  Type = octet
;  Type = prolog
;  Type = to_lower("x")
;  Type = to_upper("X")
;  Type = upper
;  false. 

The transformations use strings as the Rust API returns a string for the uppercase or lowercase of a single char. Of course, if you're using a language when this doesn't happen, you can always use:

?- char_type('X', to_upper([Upper])).
   Upper = 'X'.