Various small changes while preparing to improve type checking

[jlapeyre]

• Remove an unecessary clone and add comment
• Add some methods for asg::IndexOperator
• Rename some functions for consistency.
• Corrections in comments.
• Make some improvements in methods in types.rs Some of these methods are not completed yet. But this brings it a bit closer.
• Add equal_up_to_shape. Try to make this and equal_up_to_dims do the correct things