Auto merge of #18754 - mbrubeck:unsafe-flow, r=nox
Auto merge of #18754 - mbrubeck:unsafe-flow, r=nox

Use raw pointers instead of transmute for UnsafeFlow

This makes the code depend a bit less on rustc implementation details.

r? pcwalton

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they are refactoring only

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18754)
<!-- Reviewable:end -->

fiji-flo/servo,szeged/servo,jimberlage/servo,pyfisch/servo,thiagopnts/servo,larsbergstrom/servo,szeged/servo,pyfisch/servo,CJ8664/servo,splav/servo,mattnenterprise/servo,saneyuki/servo,splav/servo,larsbergstrom/servo,cbrewster/servo,saneyuki/servo,sadmansk/servo,KiChjang/servo,paulrouget/servo,anthgur/servo,mbrubeck/servo,paulrouget/servo,mattnenterprise/servo,ConnorGBrewster/servo,dsandeephegde/servo,anthgur/servo,eddyb/servo,emilio/servo,paulrouget/servo,avadacatavra/servo,ConnorGBrewster/servo,dsandeephegde/servo,upsuper/servo,notriddle/servo,SimonSapin/servo,dati91/servo,larsbergstrom/servo,eddyb/servo,eddyb/servo,dsandeephegde/servo,emilio/servo,DominoTree/servo,avadacatavra/servo,fiji-flo/servo,paulrouget/servo,CJ8664/servo,notriddle/servo,anthgur/servo,mattnenterprise/servo,jimberlage/servo,sadmansk/servo,nnethercote/servo,DominoTree/servo,pyfisch/servo,cbrewster/servo,nnethercote/servo,mattnenterprise/servo,fiji-flo/servo,fiji-flo/servo,KiChjang/servo,ConnorGBrewster/servo,jimberlage/servo,splav/servo,mattnenterprise/servo,nnethercote/servo,jimberlage/servo,emilio/servo,nnethercote/servo,larsbergstrom/servo,ConnorGBrewster/servo,SimonSapin/servo,jimberlage/servo,cbrewster/servo,paulrouget/servo,fiji-flo/servo,szeged/servo,rnestler/servo,thiagopnts/servo,notriddle/servo,sadmansk/servo,thiagopnts/servo,mattnenterprise/servo,emilio/servo,notriddle/servo,canaltinova/servo,pyfisch/servo,jimberlage/servo,sadmansk/servo,emilio/servo,mbrubeck/servo,peterjoel/servo,rnestler/servo,thiagopnts/servo,mattnenterprise/servo,peterjoel/servo,ConnorGBrewster/servo,peterjoel/servo,mbrubeck/servo,eddyb/servo,CJ8664/servo,DominoTree/servo,dati91/servo,peterjoel/servo,peterjoel/servo,nnethercote/servo,SimonSapin/servo,peterjoel/servo,pyfisch/servo,sadmansk/servo,splav/servo,mbrubeck/servo,SimonSapin/servo,splav/servo,pyfisch/servo,CJ8664/servo,dati91/servo,avadacatavra/servo,splav/servo,avadacatavra/servo,CJ8664/servo,DominoTree/servo,dati91/servo,canaltinova/servo,mbrubeck/servo,thiagopnts/servo,upsuper/servo,avadacatavra/servo,SimonSapin/servo,dsandeephegde/servo,ConnorGBrewster/servo,peterjoel/servo,dati91/servo,sadmansk/servo,dati91/servo,DominoTree/servo,notriddle/servo,KiChjang/servo,avadacatavra/servo,upsuper/servo,canaltinova/servo,thiagopnts/servo,splav/servo,szeged/servo,notriddle/servo,peterjoel/servo,cbrewster/servo,cbrewster/servo,KiChjang/servo,fiji-flo/servo,szeged/servo,paulrouget/servo,upsuper/servo,canaltinova/servo,larsbergstrom/servo,fiji-flo/servo,thiagopnts/servo,saneyuki/servo,cbrewster/servo,pyfisch/servo,saneyuki/servo,upsuper/servo,upsuper/servo,jimberlage/servo,paulrouget/servo,anthgur/servo,mbrubeck/servo,splav/servo,rnestler/servo,paulrouget/servo,nnethercote/servo,DominoTree/servo,pyfisch/servo,SimonSapin/servo,eddyb/servo,saneyuki/servo,cbrewster/servo,ConnorGBrewster/servo,larsbergstrom/servo,emilio/servo,SimonSapin/servo,nnethercote/servo,pyfisch/servo,canaltinova/servo,emilio/servo,notriddle/servo,nnethercote/servo,notriddle/servo,rnestler/servo,ConnorGBrewster/servo,SimonSapin/servo,eddyb/servo,CJ8664/servo,splav/servo,rnestler/servo,dsandeephegde/servo,notriddle/servo,szeged/servo,avadacatavra/servo,rnestler/servo,dsandeephegde/servo,rnestler/servo,splav/servo,pyfisch/servo,peterjoel/servo,upsuper/servo,sadmansk/servo,CJ8664/servo,larsbergstrom/servo,szeged/servo,CJ8664/servo,mattnenterprise/servo,KiChjang/servo,canaltinova/servo,canaltinova/servo,jimberlage/servo,larsbergstrom/servo,saneyuki/servo,larsbergstrom/servo,emilio/servo,avadacatavra/servo,anthgur/servo,eddyb/servo,saneyuki/servo,saneyuki/servo,saneyuki/servo,emilio/servo,eddyb/servo,saneyuki/servo,KiChjang/servo,canaltinova/servo,KiChjang/servo,fiji-flo/servo,szeged/servo,nnethercote/servo,DominoTree/servo,mbrubeck/servo,dati91/servo,sadmansk/servo,cbrewster/servo,paulrouget/servo,jimberlage/servo,larsbergstrom/servo,szeged/servo,KiChjang/servo,KiChjang/servo,notriddle/servo,rnestler/servo,DominoTree/servo,szeged/servo,anthgur/servo,dsandeephegde/servo,peterjoel/servo,DominoTree/servo,dati91/servo,anthgur/servo,KiChjang/servo,nnethercote/servo,emilio/servo,anthgur/servo,dsandeephegde/servo,upsuper/servo,mbrubeck/servo,DominoTree/servo,paulrouget/servo,thiagopnts/servo