Adding an index to monkeytype.sqlite3 speeds up stub generation

[Feuermurmel]

I've noticed that adding an index to monkeytype.sqlite3 speeds up stub generation a bit:

(venv) $ time monkeytype stub foo.bar > /dev/null

real	0m1.673s
user	0m1.242s
sys	0m0.419s
(venv) $ sqlite3 monkeytype.sqlite3
SQLite version 3.37.0 2021-12-09 01:34:53
Enter ".help" for usage hints.
sqlite> create index foo on monkeytype_call_traces (module, created_at);
sqlite> ^D
(venv) $ time monkeytype stub foo.bar > /dev/null

real	0m1.009s
user	0m0.873s
sys	0m0.104s

My monkeytype.sqlite3 is 2.6 GB in size, which resulted from running our test suite once.

[carljm]

Nice find! Would you be up for submitting a PR that adds this index to initialization of the sqlite db?