Loop Transformations: Cutting and Peeling

This MR introduces a number of loop transformations and their prequesites to the new backend.

• Add CanonicalClone pass: Clone an AST and duplicate all symbols declared inside it to retain canonicality
• Add ReshapeLoops pass collection:
  • cut_loop reimplements loop cutting like available through the old pystencils.transformations.cut_loop
  • peel_loop_front implements loop peeling, often the better alternative to cut_loop

Changes on the side:

• Move canonical duplication of symbols to the KernelCreationContext
• Add documentation for the transformations module
• Extend documentation of KernelCreationContext
• Improved AstFactory interface: introduce parse_index and clarify type hints

To Do:

• Test cases for loop cutting
• Test cases for loop peeling