style: Replace NormalDeclarationIterator return type with `impl Trait`.
style: Replace NormalDeclarationIterator return type with `impl Trait`.

Differential Revision: https://phabricator.services.mozilla.com/D3747

peterjoel/servo,notriddle/servo,sadmansk/servo,paulrouget/servo,jimberlage/servo,pyfisch/servo,larsbergstrom/servo,nnethercote/servo,peterjoel/servo,SimonSapin/servo,notriddle/servo,splav/servo,notriddle/servo,SimonSapin/servo,paulrouget/servo,peterjoel/servo,szeged/servo,DominoTree/servo,pyfisch/servo,saneyuki/servo,szeged/servo,nnethercote/servo,sadmansk/servo,saneyuki/servo,larsbergstrom/servo,jimberlage/servo,peterjoel/servo,paulrouget/servo,szeged/servo,DominoTree/servo,KiChjang/servo,jimberlage/servo,sadmansk/servo,nnethercote/servo,splav/servo,saneyuki/servo,nnethercote/servo,notriddle/servo,sadmansk/servo,sadmansk/servo,emilio/servo,pyfisch/servo,notriddle/servo,pyfisch/servo,szeged/servo,peterjoel/servo,saneyuki/servo,KiChjang/servo,larsbergstrom/servo,larsbergstrom/servo,paulrouget/servo,SimonSapin/servo,DominoTree/servo,peterjoel/servo,szeged/servo,KiChjang/servo,jimberlage/servo,DominoTree/servo,jimberlage/servo,peterjoel/servo,pyfisch/servo,szeged/servo,notriddle/servo,SimonSapin/servo,larsbergstrom/servo,jimberlage/servo,DominoTree/servo,peterjoel/servo,KiChjang/servo,jimberlage/servo,splav/servo,sadmansk/servo,sadmansk/servo,SimonSapin/servo,KiChjang/servo,paulrouget/servo,nnethercote/servo,pyfisch/servo,paulrouget/servo,emilio/servo,sadmansk/servo,emilio/servo,paulrouget/servo,pyfisch/servo,saneyuki/servo,DominoTree/servo,splav/servo,nnethercote/servo,szeged/servo,DominoTree/servo,KiChjang/servo,notriddle/servo,KiChjang/servo,splav/servo,szeged/servo,pyfisch/servo,saneyuki/servo,SimonSapin/servo,emilio/servo,saneyuki/servo,szeged/servo,nnethercote/servo,jimberlage/servo,pyfisch/servo,DominoTree/servo,szeged/servo,larsbergstrom/servo,KiChjang/servo,larsbergstrom/servo,saneyuki/servo,nnethercote/servo,emilio/servo,splav/servo,KiChjang/servo,emilio/servo,nnethercote/servo,larsbergstrom/servo,notriddle/servo,DominoTree/servo,paulrouget/servo,pyfisch/servo,emilio/servo,paulrouget/servo,jimberlage/servo,larsbergstrom/servo,larsbergstrom/servo,peterjoel/servo,DominoTree/servo,SimonSapin/servo,splav/servo,SimonSapin/servo,KiChjang/servo,splav/servo,notriddle/servo,peterjoel/servo,nnethercote/servo,emilio/servo,splav/servo,notriddle/servo,splav/servo,saneyuki/servo,saneyuki/servo,paulrouget/servo,emilio/servo,emilio/servo