Fix issue related to mixins in roundup.
Previously, mixins were only applied one level down the DFS tree while they should also be applied at the level of declaration. Makes the algorithm consistent with the version presented in the upcoming JAR paper.
src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc