The fuzzer is having a hard time discovering interesting inputs to
key_by. So far it discovered that it can do
[true].key_by({}.key_by)
which works, because {}.key_by(true) does not fail because the input is
empty, so it returns and empty dict, and then the output is {{}: true}.
But the fuzzer has not yet discovered that key_by is supposed to receive
a lambda, and it's not producing any interesting groupings.
But we do have a corpus of goldens that covers these cases. They are not
as small as the fuzz inputs, but they might be a good starting point. So
let's add a script to seed the fuzz corpus. So far I'm still not
impressed, but maybe I am just impatient and I need to let the fuzzer
run a bit longer.
