Move sapling_backingstore_get_tree_batch to cxx_bridge

[jdelliot]

Summary: X-link: https://github.com/facebookexperimental/rust-shed/pull/46

X-link: https://github.com/facebook/watchman/pull/1174

X-link: https://github.com/facebook/wangle/pull/223

X-link: https://github.com/facebook/proxygen/pull/474

X-link: https://github.com/facebook/mvfst/pull/323

X-link: https://github.com/facebook/hhvm/pull/9413

X-link: https://github.com/facebook/folly/pull/2098

X-link: https://github.com/facebook/fboss/pull/167

X-link: https://github.com/facebookincubator/velox/pull/7595

cxx.rs provides a more ergonomic and opinionated interop layer between Rust and C++ that we would like to leverage for future API chages.

This moves sapling_backingstore_get_tree_batch from the existing cbindgen implemenation to the new cxxbridge.

Reviewed By: xavierd

Differential Revision: D51321500

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51321500

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51321500

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D51321500