Project the external sequence producer callback directly into the Rust
block-delimited sequence transfer leaf. Remove the redundant C forwarding
wrapper while preserving the C-facing sequence conversion API.
Test Plan:
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; make -j1 -C tests test-zstream ZSTREAM_TESTTIME=-T2s
- ulimit -v 41943040; make -j1 -C tests fuzzer
- ulimit -v 41943040; make -j1 -C tests test-fuzzer FUZZERTEST=-T3s FUZZER_FLAGS=--no-big-tests