Sif extension

[Pialex99]

The sif extension jar must be copied into the viper_tools/backends/ directory before compiling Prusti otherwise the compilation will fail.

To generate the extension jar file:

1. copy the updated transformation from the file SIFExtendedTransformer.scala inside the source directory of the extension.
2. run the sbt package command from the extension's root directory to generate the jar file.