Improve handling of constants in datatypes

[felipebz]

Consider the code below:

declare
  const_length constant number := 5;
  var1 number(const_length);
  var2 number(5);
begin
  null;
end;

Both variables are essentially "number(5)", but currently, ZPA interprets the length of var1 as null and the length of var2 as 5.

It would be useful to improve ZPA to recognize the value of const_length and display the length of var1 as 5 as well.