Rewrite debuginfo

[marijnh]

It is currently broken as a result of the monomorphization changes. Large chunks of it have been broken since the start (only turned on with --xg). A lot of the code's assumptions just don't hold, so keeping it typechecking and partially working as one changes the rest of the compiler has been a repeated source of pain for me.

[nikomatsakis]

Not critical for 0.6; removing milestone

[jdm]

We can file more specific issues for things that are still a problem after blueibig's work here.