Fix kernel function parameters

This MR implements equality and hashing for PsSymbol such that the parameters of KernelFunctions are unique.

Also improves some error messages.

src/pystencils/backend/symbols.py

@@ -42,9 +42,21 @@ class PsSymbol:
 
     def get_dtype(self) -> PsType:
         if self._dtype is None:
-            raise PsInternalCompilerError("Symbol had no type assigned yet")
+            raise PsInternalCompilerError(
+                f"Symbol {self.name} had no type assigned yet"
+            )
         return self._dtype
 
+    def __eq__(self, other: object) -> bool:
+        return (
+            isinstance(other, PsSymbol)
+            and self._name == other._name
+            and self._dtype == other._dtype
+        )
+
+    def __hash__(self) -> int:
+        return hash((self._name, self._dtype))
+
     def __str__(self) -> str:
         dtype_str = "<untyped>" if self._dtype is None else str(self._dtype)
         return f"{self._name}: {dtype_str}"