After profiling a nixglhost hot run, it turns out that we were
spending more than 98% of the run time reading and sha256-hashing
files.
Let's give up on content hashing the files and assume that using their
name, size and last write time is good enough.
On a hot run, we reduce the run time from about 3s to 0.3s on a
nvme-powered ryzen 7 desktop.
I guess this 10x speedup probably worth the little cache corectness we
lose on the way.
Félix Baylac Jacqué