Improved Phobos dox

[bearophile]

Andrei Alexandrescu:

> http://erdani.com/d/phobos/std_array.html

One thing I'd like is for every function a link to the GitHub 
page & line of code where that Phobos function is defined. This 
is handy because I sometimes need to take a look at the 
implementation of functions I use. And I think that encourages 
people to fix the documentation faster, etc.

Another small ddoc improvement, is for each of the recently 
introduced functions a small note that tells compiler version 
number when it appeared.

Bye,
bearophile