Add custom pickling to ItemList for compactness

[mdekstrand]

This adds custom pickle logic to ItemList, so that it doesn't try to pickle the entire vocabulary when we send an item list across the wire.

[github-actions]

The GitHub 🤖 has run the tests on your PR.

Covered 100.00% of diff (coverage changed -0.01% from 92.12% to 92.11%).

origin/main...HEAD, staged and unstaged changes

• lenskit/lenskit/data/items.py (89.5%): Missing lines 346-347

Summary

• Total: 19 lines
• Missing: 2 lines
• Coverage: 89%

Source Coverage Report

Name	Stmts	Miss	Cover
lenskit-funksvd/lenskit/funksvd.py	187	8	96%
lenskit-hpf/lenskit/hpf.py	24	0	100%
lenskit-implicit/lenskit/implicit.py	94	9	90%
lenskit/lenskit/algorithms/__init__.py	67	8	88%
lenskit/lenskit/algorithms/als/__init__.py	3	0	100%
lenskit/lenskit/algorithms/als/common.py	129	2	98%
lenskit/lenskit/algorithms/als/explicit.py	121	3	98%
lenskit/lenskit/algorithms/als/implicit.py	112	1	99%
lenskit/lenskit/algorithms/basic.py	161	4	98%
lenskit/lenskit/algorithms/bias.py	150	3	98%
lenskit/lenskit/algorithms/knn/__init__.py	3	0	100%
lenskit/lenskit/algorithms/knn/item.py	303	17	94%
lenskit/lenskit/algorithms/knn/user.py	178	11	94%
lenskit/lenskit/algorithms/mf_common.py	61	0	100%
lenskit/lenskit/algorithms/ranking.py	75	11	85%
lenskit/lenskit/algorithms/svd.py	75	4	95%
lenskit/lenskit/batch/__init__.py	2	0	100%
lenskit/lenskit/batch/_predict.py	30	2	93%
lenskit/lenskit/batch/_recommend.py	46	4	91%
lenskit/lenskit/crossfold.py	136	2	99%
lenskit/lenskit/data/__init__.py	8	0	100%
lenskit/lenskit/data/checks.py	37	0	100%
lenskit/lenskit/data/dataset.py	364	19	95%
lenskit/lenskit/data/fetch.py	38	28	26%
lenskit/lenskit/data/items.py	147	3	98%
lenskit/lenskit/data/matrix.py	115	5	96%
lenskit/lenskit/data/movielens.py	96	18	81%
lenskit/lenskit/data/mtarray.py	57	3	95%
lenskit/lenskit/data/tables.py	25	0	100%
lenskit/lenskit/data/vocab.py	88	7	92%
lenskit/lenskit/diagnostics.py	4	0	100%
lenskit/lenskit/math/__init__.py	0	0	100%
lenskit/lenskit/math/solve.py	6	0	100%
lenskit/lenskit/metrics/__init__.py	0	0	100%
lenskit/lenskit/metrics/predict.py	32	0	100%
lenskit/lenskit/metrics/topn.py	212	1	99%
lenskit/lenskit/parallel/__init__.py	4	0	100%
lenskit/lenskit/parallel/chunking.py	20	1	95%
lenskit/lenskit/parallel/config.py	65	8	88%
lenskit/lenskit/parallel/invoker.py	31	2	94%
lenskit/lenskit/parallel/pool.py	54	9	83%
lenskit/lenskit/parallel/sequential.py	22	0	100%
lenskit/lenskit/parallel/serialize.py	51	1	98%
lenskit/lenskit/parallel/worker.py	43	3	93%
lenskit/lenskit/topn.py	109	25	77%
lenskit/lenskit/util/__init__.py	72	19	74%
lenskit/lenskit/util/envcheck.py	57	44	23%
lenskit/lenskit/util/logging.py	19	0	100%
lenskit/lenskit/util/random.py	26	3	88%
lenskit/lenskit/util/test.py	103	19	82%
lenskit/lenskit/util/timing.py	28	0	100%
TOTAL	3890	307	92%