Added copy constructors to "Programming in D"

[Ali Çehreli]

1) After about three years, I finally added copy constructors:

 
http://ddili.org/ders/d.en/special_functions.html#ix_special_functions.copy%20constructor

Of course, now I stress that postblit is discouraged.

2) The other noteworthy change in the book is my now-different stance on 
variables: Now I recommend 'const' over 'immutable' for variables. And I 
favor 'in' for parameters over 'const'. (Unfortunately, not all code in 
the book follow those guidelines.)

And I recommend the -preview=in command line switch:

 
http://ddili.org/ders/d.en/function_parameters.html#ix_function_parameters.in,%20parameter

3) I've updated all electronic formats: PDF, EPUB, AZW3, and MOBI:

   http://ddili.org/ders/d.en/

I had some trouble with ebooks in the past: The fonts would not work and 
table of contents would not be generated. Please let me know if you see 
something wrong.

WARNING: The paper book publishers are not updated yet. If you order a 
paper book now, you will get the old book. (I think from December 2020.)

Ali