DefPathData should use InternedString, not Name

[nikomatsakis]

Currently DefPathData embeds a Name. The intention was to have an interned string and ignore hygiene. But it turns out that Name still has a kind of quasi-hygiene -- using gensym, you can create two names with the same string representation but distinct identifiers. We should be using InternedString in DefPathData instead, I believe. Otherwise you can wind up with two def-ids with def-paths that, after serialization, appear exactly the same (since we serialize the Name by writing out the string value, but that is not sufficient to capture this gensym quality).

This shows up e.g. in the iter variable created by the for lowering.

[michaelwoerister]

cc me

[eddyb]

We can also get rid of gensym and use hygiene instead.

[nikomatsakis]

PS I have a fix for this on One Of My Branches