rustc: map node ids through a table that ensures bitset indexes in dataflow are dense
auto merge of #7202 : pcwalton/rust/resolve-speed, r=pcwalton

I'm pretty sure this got adequately reviewed by a combination of @nikomatsakis and @graydon so I'm rubber stamping it because I really want this in (it makes it much less likely for me to swap, allowing me to get more work done).