This predicate returns true if the named ML constant has been hidden by
the function hide_constant; it returns false if the constant is not hidden.
Hiding a constant forces the quotation parser to treat the constant as
a variable (lexical rules permitting).
FAILURE CONDITIONS
Never fails.
EXAMPLE
# is_hidden "SUC";;
val it : bool = false
# hide_constant "SUC";;
val it : unit = ()
# is_hidden "SUC";;
val it : bool = true