trait types should require explicit pointer types

[brson]

Currently when you use a trait foo as a type it really means @foo. Like closures and vectors we should have more control over memory. Being able to cast to an &foo without copying would immediately make this style of polymorphism more useful.

Under this scheme you would probably be required to write a trait type as either @foo, ~foo, or &foo, and foo would be disallowed.

Simalarly, it would be useful if you could cast from a boxed type to a boxed trait without copying. That's probably a different issue though.

[jruderman]

Related to #3157

[nikomatsakis]

I would go so far as to say it's a dup.