Built-in trait inheritance

[brson]

Need to be able to trait Foo: Copy { }.

This will need checks to verify that types implementing kinds are actually that kind.

[catamorphism]

Important, but won't get done for 0.6. De-milestoning

[catamorphism]

Nominating for milestone 2, backwards-compatible

[nikomatsakis]

This could be important for #6308, since I imagine some traits will want to extend Sized

[graydon]

accepted for feature-complete milestone

[thestinger]

This is implemented and there are tests for it in run-pass.