Mixin template evaluated to string can convert to string mixin expression implicitly

[kenji hara]

Issue in bugzilla:
  http://d.puremagic.com/issues/show_bug.cgi?id=6207
Test implementation:
  https://github.com/9rnsr/dmd/commit/52e4491cf6bf9ccf6dccc1fa60581fd6797d39b3

String mixin feature in D is very useful, but its syntax is bit ugly.
My proposal will make it more softly, and introduces more generative
programming.

Any thoughts?

Kenji