Add isCopyConstructible to std.traits

[Rene Zwanenburg]

I need a template constraint to ensure a struct is not copyable. 
While I found hasElaborateCopyConstructor* in std.traits, there 
seems not to be something like isCopyConstructible. Shall I 
create an enhancement request or did I miss something? Same 
question for isDefaultConstructible.

* This site and TDPL both try to make a clear distinction between 
C++'s copy constructor and D's postblit. Wouldn't 
hasElaboratePostblit be a better name?