Avoid undefined behavior when subtracting tagged pointers (#715)

When compiling with HardwareAddressSanitizer, the top byte of pointers
are tagged, hence subtracting two pointers may overflow. Overflow is
undefined behavior for signed integers, resulting in errors such as:
    third_party/nanobind/src/nb_type.cpp:123:50: runtime error: signed integer overflow: -6701078944169102656 - 9079534012674239392 cannot be represented in type 'intptr_t' (aka 'long')

This patch fixes the issue by using unsigned integers, for which
overflow is well-defined.

src/nb_type.cpp

@@ -124,7 +124,10 @@ PyObject *inst_new_ext(PyTypeObject *tp, void *value) {
     }
 
     // Compute offset to instance value
-    int32_t offset = (int32_t) ((intptr_t) value - (intptr_t) self);
+    // Use uint64_t because subtracting tagged pointers (e.g., with
+    // HardwareAddressSanitizer) may overflow, which is undefined behavior for
+    // signed integers.
+    int32_t offset = (int32_t) ((uintptr_t) value - (uintptr_t) self);
 
     bool direct = (intptr_t) self + offset == (intptr_t) value;
     if (NB_UNLIKELY(!direct)) {