Track span info for type constraints

[catamorphism]

tydecode::parse_constr currently uses dummy spans for type constraints because (I think) spans in types don't get serialized right now. Fix that.

[marijnh]

I'd hold off working on this until we actually decide to keep these around.

[nikomatsakis]

Not relevant as of commit 41a21f0, which removed current incarnation of typestate.