B0_memo_logSourceMemo log.
A log has all the build operations, the hashed file paths and a few global timings.
file_hashes l has all the files that were hashed through the memo.
hash_dur l is the time span spent hashing.
total_dur l is the time spanning from B0_memo.make to of_memo.
cpu_dur l is the CPU time spanning from B0_memo.make to of_memo.
jobs l is the maximal number of concurent spawns given to the build operation executor.
ops l are the operations of the log.
bincode is a binary codec for logs.
write f l writes log l to file f.
read f read a log from file f.