Revert previous front-end API changes + Cherry-pick fixes from master

This MR reverts changes previously applied to the front-end API, in order to minimize divergence on the front-end side when the new backend is introduced.

This MR furthermore reapplies !377 (merged) and !382 (merged) onto the development branch.

src/pystencils/backend/kernelcreation/analysis.py

@@ -9,7 +9,8 @@ import sympy as sp
 from .context import KernelCreationContext
 
 from ...field import Field
-from ...sympyextensions import Assignment, AssignmentCollection
+from ...assignment import Assignment
+from ...simp import AssignmentCollection
 
 from ..exceptions import PsInternalCompilerError, KernelConstraintsError