Split up loopy.kernel. Clean up kernel creation.

loopy/__init__.py

loopy/check.py

loopy/codegen/__init__.py

loopy/codegen/bounds.py

loopy/codegen/control.py

loopy/codegen/expression.py

loopy/codegen/loop.py

loopy/compiled.py

loopy/cse.py

loopy/kernel/__init__.py

loopy/kernel/creation.py

loopy/kernel/data.py

loopy/kernel/tools.py

loopy/padding.py

loopy/preprocess.py

loopy/schedule.py

loopy/subst.py

loopy/symbolic.py