Add caching for simple __init__ functions.

[DavidCEllis]

For generated __init__ functions that don't include pre/post-init, default factories or non-init defaults, the __init__ functions are now cached based on element number.

This also breaks __init__.py up into __init__.py, constants.py, functions.py and methods.py as it had gotten overly large and was getting annoying to navigate. constants and functions exist partly to avoid circular imports with __init__. All of the method making logic now lives in methods.py.