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).
Revert "stop using an absolute rpath"

This reverts commit 708395d65d689080cc96dd091adab3fc4c5c3f27.
auto merge of #7192 : brson/rust/rpath, r=thestinger

r? @thestinger

This reverts commit 708395d65d689080cc96dd091adab3fc4c5c3f27.

Fixes #7191.