equivariant functions

[Robert Fraser]

Andrei Alexandrescu wrote:
> Yah, that's where the PassQual template is needed. I'd agree without joy 
> that, if handling true equivariance is too unwieldy, we can resign to 
> solving only equivariance in qualifier.

FWIW, people have done without type eqivariance w/o complaint since 
static typing was invented. People have been complaining about const 
equivariance since at least C++ and D makes it worse.