new DIP40: Template parameter deduction for constructors

[Timothee Cour]

A proposed feature of C++14 is to introduce template parameter
deduction for constructors, see paper, mentioned here. The idea is to
deduce template parameters when calling a constructor given the
arguments given to the constructor, whenever possible. A compile error
occurs when the deduction is ambiguous. The benefits would be:
* make the code more DRY
* make boilerplate of class instantiators unnecessary in most cases
(they're all over phobos, eg: std.typecons.tuple,
std.typecons.rebindable etc)
* make D more consistent: it deduces template parameters for
functions, so why not for constructors, when this is unambiguous?
* it won't break any code.
Note, just as for deduction of normal functions, it should work with 0
or more template parameters specified (ie the first k>=0 templates may
be provided).