Using rust coreutils instead of busybox

[arihant2math]

I've been playing around with cross-compiling rust programs to moss and coreutils fares well. It seems like targeting musl is a requirement for now.

Most utils seem to work properly, ls -la/ls -l seems to crash due to listxattr not being implemented, but that's probably an easy fix.

[hexagonal-sun]

Indeed, the current WakerSet doesn't support selective wakeups. You're correct that we will need a new primitive to support this.

Regarding switching to threading for usertest: while I think it's a nice test for exercising our futex implementation, I think having process isolation helps to prevent side-effect contamination.

[arihant2math]

Wrong issue, but yes, I think keeping it process-per-test is good. I'll create a new primitive.