Support type annotation in WAC

[salmans]

When instantiating a component, it's useful to allow the user to provide type annotation that fixes the component's expected imports and exports. For example, in the scenario described in #56, the type annotation can be used to suppress the unknown import error when the import is intended but is not present in the component's WIT, generated by witgen.

[peterhuene]

Proposed syntax might be:

new-expr                ::= 'new' package-name type-ascription? '{' instantiation-args '}'
type-ascription         ::= ':' (id | package-path)

Constraints:

1. The id or package-path must name a world.
2. The result of the new expression must be a subtype of the given world.
3. The ascribed type will be used for validation of the instantiation args.

[peterhuene]

The : delimiter might be confusing since this would be legal with the above grammar: new foo:bar:foo:bar/baz {}.

Perhaps as instead of :, leading to new foo:bar as foo:bar/baz {}.