Revised Array Modelling & Memory Model

This MR revises the way C arrays, as well as heap memory and pointers, are modelled and translated by pystencils.

Changes concerning arrays:

• Change PsArrayType to receive a fixed, n-dimensional, nonempty shape, and a non-const, non-array element type
• Change PsSubscript to only work on arrays via an n-dimensional index tuple
• Change PsArrayInitList to an n-dimensional matrix of expressions
• Add contextual and default typing for array declarations and inline array literals

Changes concerning pointers and memory:

• Remove PsDeref
• Introduce PsMemAcc to access memory by pointers
• Assume all non-field raw pointers to be aliases of each other, adapt loop-invariant code motion accordingly
• Introduce mem_acc front-end function for accessing memory by pointer in SymPy

Changes to the typifier:

• Remove constness handling from the type context; only work with non-const data types while processing expressions
• In PsAssignment, use a seperate algorithm to determine the mutated memory object and its constness