Kind checker doesn't check that struct fields are copyable

[catamorphism]

kind::check_expr just doesn't have a case for structs, so you can write:

B { foo: x }

if x has type ~T and B is a struct with a ~T field foo.

This wasn't showing up before because last-use was enabled and most values involved in initializing a struct are going to be last uses.

I have a fix, testing it.

[brson]

Nice. I think I hit this today.

[catamorphism]

This was painful to track down. I think it's the root cause of everything breaking when I disabled last-use, but I'm still waiting for the build to finish.

[nikomatsakis]

Good catch.