Use memfd for file data

[Xyene]

We can create a memfd pointing to decompressed problem data, and pass that off to the submissions.

Pros:

• no more piping necessary in judge; fewer context switches as a result
• submissions can seek the fd
• still no disk thrashing

Cons(?):

• 3.7+ kernel necessary
• extra memcpy

[quantum5]

Support for creation of memfd-backed files is implemented in #836, with unlinked temporary files as fallback on FreeBSD.

This should now be a matter of writing decompressed data into an MemoryIO instance and passing the fd to TracedPopen.