Fix handling of constness in Typifier

Improves the handling of constness in the typifier.

Left-hand sides of non-declaration assignments may not be const. For the typifier to tell apart const from non-const expressions, it needs to track constness in the TypeContext. This MR adds that ability: All expressions are now by default assigned const types, unless otherwise specified. This does not change the handling of untyped symbols, which still receive the default_dtype. A PsSymbolExpr may therefore now have a different const qualifier than its wrapped PsSymbol.

Additionally, this MR further documents and clarified the behaviour and control flow of the typifier.

src/pystencils/backend/kernelcreation/typification.py

@@ -184,6 +184,11 @@ class TypeContext:
         expr.dtype = self._target_type
 
     def _compatible(self, dtype: PsType):
+        """Checks whether the given data type is compatible with the context's target type.
+        
+        If the target type is ``const``, they must be equal up to const qualification;
+        if the target type is not ``const``, `dtype` must match it exactly.
+        """
         assert self._target_type is not None
         if self._target_type.const:
             return constify(dtype) == self._target_type