The implementation essentially desugars during type collection and AST
type conversion time into the parameter scheme we have now. Only fully
qualified names--e.g. `<T as Foo>::Bar`--are supported.
This code fits better in check because it is checking that the impl
matches the interface. This allows us to avoid the awkward constructions
that lazilly collect traits and so forth. It also permits us to make
use of the results of variance inference.
Introduce syntax like:
iface animal { ... }
class cat implements animal { ... }
to allow classes to implement ifaces. Casting classes to ifaces
is *not* yet supported. ifaces that a class implements are not
yet included in metadata.
The syntax is subject to change, and may go away completely if we
decide to use duck typing to relate classes with ifaces (see
http://smallcultfollowing.com/babysteps/blog/2012/04/10/declared-vs-duckish-typing/ )
