Consider putting the operations in their own library crate

[nox]

[tarka]

I'm open to this, especially some of the utility operations that may be useful elsewhere. Currently I'm doing a long refactor to support more fine-grained parallelism, which means I'm rewriting this code a lot. I'll revisit once it's stabilised.

[tarka]

Low-level sparse file handling and related operations have been published as libfs