Rewrite bcachefs in Rust

[link2xt]

From https://lore.kernel.org/linux-bcachefs/slvis5ybvo7ch3vxh5yb6turapyq7hai2tddwjriicfxqivnpn@xdpb25wey5xd/

A lot of codebase modernization has been happening over the past six months, to prepare for Rust. With the latest features recently available in C and in the kernel, we can now do incremental refactorings to bring code steadily more in line with what the Rust version will be, so that the future conversion will be mostly syntactic - and not a rewrite. The big enabler here was CLASS(), which is the kernel's version of pseudo-RAII based on __cleanup(); this allows for the removal of goto based error handling (Rust notably does not have goto).