[dajiaohuang]

Fixes #691.

Easy2::default_configure previously derived libcurl callback userdata by casting a shared reference to Inner into a mutable pointer. The callback later mutates the handler through that pointer, and the stored pointer can also outlive the temporary borrow and be invalidated as Easy2 moves.

Keep Inner in an owning raw-pointer wrapper created with Box::into_raw, and pass that allocation pointer to libcurl callbacks. The wrapper provides the existing field access and reconstructs the Box on drop, after curl_easy_cleanup has run. Add a synthetic callback regression that moves the owner and invokes the write callback through the saved userdata.

Validation:

• cargo test --offline --package curl --lib --no-default-features — 6 passed.
• rustfmt --check --edition 2018 src/easy/handler.rs — passed.
• git diff --check — passed.
• Miri was unavailable on the installed Windows toolchain; the issue includes the Miri failure, with an independent BorrowSanitizer confirmation.