Before 2011 it was recommended to use a prime number for the initial
size. In 2011 the OCaml hash table was reimplemented using a hash
function based on Murmur 3. Hashtbl.create now adjusts the initial
size to the next power of 2 (minimum 16). So replace obsolete
'Hashtbl.create 13' with 'Hashtbl.create 16'.
Run this command across the source:
perl -pi.bak -e 's/(20[012][0-9])-20[12][012]/$1-2023/g' `git ls-files`
and remove changes to po{,-docs}/*.po{,t} (these will be regenerated
later when we run 'make dist').
Isolate the logic for the memoized disk cache in a small module, so it
can be reused for other tools.
Other than refactoring, there should be no behaviour changes.
