Turbopack: bincode: Use bincode to store the contents of value cells (#86338)
This PR switches `turbo-tasks-backend` to use `bincode` instead of serde for cell contents. To keep PR size manageable, `TaskInput`s are still serialized using serde and a compatibility shim. The next PR fixes that.
See https://github.com/vercel/next.js/pull/86631 for benchmark numbers.