Make change_size function safer (sizing_2008)

[oswald3141]

Changes to change_size in this PR:

• allow an input vector to have any bits direction (downto and to);
• add checks preventing an ill-formed resize operation;
• clarify MSB position in change_size (in comments).

However, there's a small problem with it. Line 299 now may cause a compilation warning (null range). The purpose of this line is to trigger a compilation error at synthesis stage (line 305) even if a synthesis tool ignored the assert on line 304. As a bonus, the error will be triggered on the line with the erroneous user's code, and not in sizing_2008.vhdl.
If you know a way of avoiding the warning, please, let me know.

I was also thinking about moving the change_size functions into a separate file. Their purpose seems now to be too far from the other functions in sizing_2008. It was probably a mistake from me to add these functions in this file. Please, let me know what you think.