feat: add loom::cell::Cell::into_inner

[jbr]

Thanks for loom! I don't have sufficient context to know if there is a reason for this absence, but I ran into this difference between std::cell::Cell and loom::cell::Cell and it seemed easy enough to fix

[taiki-e]

Thanks.

[notgull]

Any chance this change can be fit into a loom release?

[taiki-e]

Already released in 0.7.2.
https://github.com/tokio-rs/loom/blob/master/CHANGELOG.md#072
https://docs.rs/loom/latest/loom/cell/struct.Cell.html#method.into_inner