Include the struct type in frozen hash implementation

This modifies the provided hash method to also include the struct type.
This improves hash quality for mappings or sets that contain multiple
different struct types as keys, as structs of different types with the
same data will now hash differently, better matching the `__eq__`
implementation.

As an implementation detail, structs should now hash as a tuple of the
type and all following elements.

```python
from msgspec import Struct

class Point(Struct, frozen=True):
    x: int
    y: int

assert hash(Point(1, 2)) == hash((Point, 1, 2))
```

msgspec/_core.c

@@ -7140,19 +7140,26 @@ Struct_hash(PyObject *self) {
         return PyObject_HashNotImplemented(self);
     }
 
+    /* First hash the type by its pointer */
+    size_t type_id = (size_t)((void *)st_type);
+    /* The lower bits are likely to be 0; rotate by 4 */
+    type_id = (type_id >> 4) | (type_id << (8 * sizeof(void *) - 4));
+    acc += type_id * MS_HASH_XXPRIME_2;
+    acc = MS_HASH_XXROTATE(acc);
+    acc *= MS_HASH_XXPRIME_1;
+
+    /* Then hash all the fields */
     nfields = StructMeta_GET_NFIELDS(Py_TYPE(self));
-
     for (i = 0; i < nfields; i++) {
-        Py_uhash_t lane;
         val = Struct_get_index(self, i);
         if (val == NULL) return -1;
-        lane = PyObject_Hash(val);
-        if (lane == (Py_uhash_t)-1) return -1;
-        acc += lane * MS_HASH_XXPRIME_2;
+        Py_uhash_t item_hash = PyObject_Hash(val);
+        if (item_hash == (Py_uhash_t)-1) return -1;
+        acc += item_hash * MS_HASH_XXPRIME_2;
         acc = MS_HASH_XXROTATE(acc);
         acc *= MS_HASH_XXPRIME_1;
     }
-    acc += nfields ^ (MS_HASH_XXPRIME_5 ^ 3527539UL);
+    acc += (1 + nfields) ^ (MS_HASH_XXPRIME_5 ^ 3527539UL);
     return (acc == (Py_uhash_t)-1) ?  1546275796 : acc;
 }
 

tests/test_struct.py

@@ -1460,6 +1460,16 @@ def test_frozen_hash_mutable_objects_hash_errors(self):
         with pytest.raises(TypeError, match="unhashable type"):
             hash(p)
 
+    def test_hash_includes_type(self):
+        Ex1 = defstruct("Ex1", ["x"], frozen=True)
+        Ex2 = defstruct("Ex2", ["x"], frozen=True)
+        Ex3 = defstruct("Ex3", [], frozen=True)
+        Ex4 = defstruct("Ex4", [], frozen=True)
+        assert hash(Ex1(1)) == hash(Ex1(1))
+        assert hash(Ex1(1)) != hash(Ex2(1))
+        assert hash(Ex3()) == hash(Ex3())
+        assert hash(Ex3()) != hash(Ex4())
+
     @pytest.mark.parametrize("base_gc", [True, None, False])
     @pytest.mark.parametrize("base_frozen", [True, False])
     @pytest.mark.parametrize("has_gc", [True, None, False])