C++ 20 modules

[Nathan Sidwell]

I've completed merging modules to trunk modulo the testsuite.  I 
included a few smoke tests, but nothing more.  I'll add the rest when 
the smoke clears.

There will undoubtedly be issues related to configs that I've not built. 
  As I mentioned I did what I could.

Further, it'll undoubtedly break when you go poking at it, as we all 
have unique and special sticks to poke with.  This might disappoint. 
But just think what had to go right to get you there :)  Recently these 
kinds of bugs have been simple to fix -- a thinko or simply forgetting 
about a piece of C++.  Please file issues in bugzilla.

There are some known omissions listed in the documentation (go read it).

Finally, thanks to the early testers, those that supported me in 
implementing such an exciting feature, and those who listened to my 
comments and suggestions in many WG21 meetings.

nathan

-- 
Nathan Sidwell