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

@@ -304,10 +304,10 @@ class Typifier:
                 if ctr.symbol.dtype is None:
                     ctr.symbol.apply_dtype(self._ctx.index_dtype)
 
-                tc_lhs = TypeContext(ctr.symbol.dtype)
-                self.visit_expr(start, tc_lhs)
-                self.visit_expr(stop, tc_lhs)
-                self.visit_expr(step, tc_lhs)
+                tc_index = TypeContext(ctr.symbol.dtype)
+                self.visit_expr(start, tc_index)
+                self.visit_expr(stop, tc_index)
+                self.visit_expr(step, tc_index)
 
                 self.visit(body)