fix(oss): Use the native iobuf now published by folly

[ben--]

This diff enables a number of references to folly::iobuf that required a rust <> native bridge.

It SHOULD NOT be landed until folly iobuf support lands.

Tested through building Sapling/EdenFS (requires other fixes to run clean):

act --container-architecture linux/x86_64 -W .github/workflows/edenfs_linux.yml

[facebook-github-bot]

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