C++: Fix ABI mismatch with Slice on MSVC AArch64

The second constructor of `Slice` causes the Slice to be passed
differently (in different register) as a return value.

So remove that constructor and use a helper function to
construct a Slice

api/cpp/include/slint_string.h

@@ -3,6 +3,7 @@
 
 #pragma once
 #include <string_view>
+#include <span>
 #include "slint_string_internal.h"
 
 namespace slint {
@@ -223,11 +224,26 @@ private:
 };
 
 namespace private_api {
+
+template<typename T>
+inline cbindgen_private::Slice<T> make_slice(const T *ptr, size_t len)
+{
+    return cbindgen_private::Slice<T> {
+        // Rust uses a NonNull, so even empty slices shouldn't use nullptr
+        .ptr = ptr ? const_cast<T *>(ptr) : reinterpret_cast<T *>(sizeof(T)),
+        .len = len,
+    };
+}
+
+template<typename T, size_t Extent>
+inline cbindgen_private::Slice<std::remove_const_t<T>> make_slice(std::span<T, Extent> span)
+{
+    return make_slice(span.data(), span.size());
+}
+
 inline cbindgen_private::Slice<uint8_t> string_to_slice(std::string_view str)
 {
-    return cbindgen_private::Slice<uint8_t> {
-        const_cast<unsigned char *>(reinterpret_cast<const unsigned char *>(str.data())), str.size()
-    };
+    return make_slice(reinterpret_cast<const uint8_t *>(str.data()), str.size());
 }
 }