Make object.Object @trusted

This pull request makes methods of `object.Object` trusted or (if possible) safe.

I think it is very important to make Phobos safe.
For example, now `std.conv.to` cannot be made safe due to this issue, and
therefore other functions which uses `std.conv.to` cannot be made safe.
And `std.conv.to` is used in everywhere in Phobos.