Round 2 of review:
Round 2 of review:

* Converted pre-existing trace!s back from debug!
* Computed `cur` from the top-most frame instead of storing it.
* Wrapped to 80 columns.
* Removed prev_active and all_candidates from `BacktrackStack`
* Removed unnecessary type annotations.
* Called activation_error() only when starting to backtrack, since it bailed
  early when passed an error anyway.

tsurai/cargo,Frederick888/cargo,sbeckeriv/cargo,fhahn/cargo,ArtemGr/cargo,Kimundi/cargo,Kimundi/cargo,gchp/cargo,AndiDog/cargo,aturon/cargo,winger/cargo,ThinTim/cargo,blaenk/cargo,blaenk/cargo,aturon/cargo,eminence/cargo,shaleh/cargo,cesarb/cargo,winger/cargo,vojtechkral/cargo,cmr/cargo,iliekturtles/cargo,ArtemGr/cargo,blaenk/cargo,AndiDog/cargo,ArtemGr/cargo,durka/cargo,tshepang/cargo,achanda/cargo,ThinTim/cargo,akiss77/cargo,aturon/cargo,ThinTim/cargo,Frederick888/cargo,mark-buer/cargo,rust-lang/cargo,brson/cargo,winger/cargo,shaleh/cargo,gchp/cargo,cmr/cargo,shaleh/cargo,Kimundi/cargo,rustyhorde/cargo,mark-buer/cargo,vojtechkral/cargo,AndiDog/cargo,gchp/cargo,brson/cargo,winger/cargo,blaenk/cargo,ThinTim/cargo,vi/cargo,fhahn/cargo,durka/cargo,AndiDog/cargo,mark-buer/cargo,ThinTim/cargo,iliekturtles/cargo,huonw/cargo,rust-lang/cargo,huonw/cargo,tsurai/cargo,fhahn/cargo,blaenk/cargo,vi/cargo,tshepang/cargo,nipunn1313/cargo,achanda/cargo,tsurai/cargo,iliekturtles/cargo,iliekturtles/cargo,fhahn/cargo,aturon/cargo,iliekturtles/cargo,Frederick888/cargo,gchp/cargo,nipunn1313/cargo,aturon/cargo,sbeckeriv/cargo,nipunn1313/cargo,eminence/cargo,eminence/cargo,rustyhorde/cargo,cesarb/cargo,sbeckeriv/cargo,gchp/cargo,tsurai/cargo,AndiDog/cargo,akiss77/cargo,rustyhorde/cargo,vi/cargo,shaleh/cargo,eminence/cargo,cesarb/cargo,huonw/cargo,tshepang/cargo,Kimundi/cargo,winger/cargo,brson/cargo,mark-buer/cargo,vojtechkral/cargo,durka/cargo,Frederick888/cargo,achanda/cargo,Frederick888/cargo,cesarb/cargo,vi/cargo,durka/cargo,shaleh/cargo,Kimundi/cargo,tshepang/cargo,ArtemGr/cargo,akiss77/cargo,akiss77/cargo,tsurai/cargo,huonw/cargo,cmr/cargo,achanda/cargo,rust-lang/cargo,brson/cargo,rustyhorde/cargo,fhahn/cargo,akiss77/cargo,brson/cargo,huonw/cargo,sbeckeriv/cargo,cesarb/cargo,ArtemGr/cargo,cmr/cargo,vi/cargo,nipunn1313/cargo,mark-buer/cargo,vojtechkral/cargo,rust-lang/cargo,cmr/cargo