change mixins

[Walter Bright]

Right now, mixins are defined and used as:

    template foo(T) { declarations... }

    mixin foo!(int) handle;

The proposal is to switch it around:

    mixin template foo(T) { declarations... }

    foo!(int) handle;

to follow the notion that mixin templates are very different from 
regular templates, and that should be reflected in their definition 
rather than use.

What do you think?