implicit construction operator

[ketmar]

aliak wrote:

> It makes libraries *much* more intuitive and expressive (C++ just got it 
> wrong by choosing the wrong default). If you allow library authors to opt 
> in instead of opt out then it becomes a conscious decision for one.

library authors can create overloads for various argument types. with 
`foo(T...) (T args)` it is possible to generate conversions with CTFE. so 
we *already* have such feature.