Restructure transforms code

loopy/__init__.py

loopy/kernel/tools.py

loopy/preprocess.py

loopy/transform/__init__.py

loopy/transform/arithmetic.py

loopy/transform/array_buffer_map.py

loopy/transform/buffer.py

loopy/transform/data.py

loopy/transform/diff.py

loopy/transform/fusion.py

loopy/transform/ilp.py

loopy/transform/iname.py

loopy/transform/instruction.py

loopy/transform/padding.py

loopy/transform/precompute.py

loopy/transform/subst.py

test/test_diff.py