Refactor Field Modelling

This MR introduces a significant refactoring of the backend handling of fields. This comprises several parts:

Symbol Properties

• Introduce a system of symbol properties used to add arbitrary information to symbols
• Remove subclasses for field-associated symbols
• Introduce properties for marking field shape, stride, and pointer symbols
• Refactor kernel parameters to hold symbol properties; remove field-specific KernelParameter subclasses

Field Buffers

• Rename PsLinearizedArray to PsBuffer
• move PsBuffer and PsSymbol to module backend.memory
• Adapt PsBuffer constructor to take all its pointer, shape and stride as PsSymbol or PsConstant

Buffers in the AST

• Replace PsArrayAccess by PsBufferAcc; PsBufferAcc takes an nd index tuple instead of a linearized index
• Adapt freeze and typification to the above
• Introduce LowerToC transformation pass which lowers buffer accesses to raw memory accesses
• Subsume EraseAnonymousStructs into LowerToC

Kernel Translation

• Add LowerToC pass and late canonicalization and constant folding passes to the create_kernel pipeline

Bugfixes

A number of bugs revealed during development are being fixed:

• clone() on expression nodes used to forget the nodes' data type; now keeps it
• RHS expression in array declarations was not assigned a data type if the type was dictated by the LHS

Closes #101 (closed)

Continues work from !408 (merged)