Change symbol typification: Infer unknown LHS data types from RHS

This MR extends the typifier to infer the data type of untyped LHS symbols from an Assignment's RHS.

This feature is required when pystencils extracts subexpressions of non-default type, such as long boolean expressions from piecewise definitions.

Also, a few additional test cases for the typifier are added.

src/pystencils/backend/kernelcreation/context.py

@@ -12,7 +12,7 @@ from ...sympyextensions.typed_sympy import TypedSymbol
 
 from ..symbols import PsSymbol
 from ..arrays import PsLinearizedArray
-from ...types import PsType, PsIntegerType, PsNumericType, PsScalarType, PsStructType
+from ...types import PsType, PsIntegerType, PsNumericType, PsScalarType, PsStructType, deconstify
 from ..constraints import KernelParamsConstraint
 from ..exceptions import PsInternalCompilerError, KernelConstraintsError
 
@@ -63,8 +63,8 @@ class KernelCreationContext:
         default_dtype: PsNumericType = DEFAULTS.numeric_dtype,
         index_dtype: PsIntegerType = DEFAULTS.index_dtype,
     ):
-        self._default_dtype = default_dtype
-        self._index_dtype = index_dtype
+        self._default_dtype = deconstify(default_dtype)
+        self._index_dtype = deconstify(index_dtype)
 
         self._symbols: dict[str, PsSymbol] = dict()