Give type parameters kind constraints, and check them.

[froystig]

Some items need to have information about the possible kind of the type parameters they accept. For instance, the following function:

fn vec1[copy T](&T x) -> vec[T] { ret vec(x); }

The typechecker should check that kind constraints are satisfied whenever type arguments are actually applied to items. Middle-end should make sure to use the kind constraint when calculating the kind of a type-parameter type.

[graydon]

Shifted to rustc.

[graydon]

Subsumed into #234, which is now wip. Closing this.