Make assignments to linear indices work

loopy/check.py

loopy/codegen/instruction.py

loopy/kernel/__init__.py

loopy/kernel/creation.py

loopy/kernel/data.py

loopy/maxima.py

loopy/preprocess.py

loopy/symbolic.py

loopy/target/c/__init__.py

loopy/transform/arithmetic.py

loopy/transform/buffer.py

loopy/transform/diff.py

loopy/transform/subst.py

test/test_loopy.py

test/test_numa_diff.py