Coherence rules are stricter than they need to be

[nikomatsakis]

The current orphan rules in the coherence check are stricter than they need to be. For example, if I have

struct Foo

then I cannot have an impl like the following

impl ToStr for @Foo

The goal of the orphan rules is to guarantee that we can detect duplicate trait implementations so as to enforce coherence. To this end, it is only necessary to ensure that we could find any potentially conflicting impl. If we enforce the rule that the "for" type must "involve" some nominal type from the current crate, that is adequate for this purpose. For example @Foo should be ok but also Option<Foo>.

[nikomatsakis]

I ran into this because I have a branch that implements Encodable for ty::t, but it doesn't work because this is *t_opaque

[nikomatsakis]

Fixed.