Support functions with multiple return values

doc/ref_kernel.rst

doc/ref_transform.rst

loopy/__init__.py

loopy/codegen/instruction.py

loopy/expression.py

loopy/kernel/__init__.py

loopy/kernel/creation.py

loopy/kernel/data.py

loopy/kernel/tools.py

loopy/library/function.py

loopy/library/reduction.py

loopy/maxima.py

loopy/preprocess.py

loopy/schedule.py

loopy/statistics.py

loopy/symbolic.py

loopy/target/__init__.py

loopy/target/c/__init__.py

loopy/target/c/codegen/expression.py

loopy/target/opencl.py