Matroid serialisation and matroid encodings #15391

Job	Run time
doctest (1.10, ubuntu-latest)	24m 18s
doctest (1.11-nightly, ubuntu-latest)	22m 46s
test (1.6, short, ubuntu-latest)	1h 9m 25s
doctest (nightly, ubuntu-latest)	4m 57s
doctest (1.10, ubuntu-latest, depwarn=error)	24m 18s
test (1.6, long, ubuntu-latest)	54m 47s
doctest (1.10, macOS, RPTU)	11m 15s
test (1.10, short, ubuntu-latest)	56m 34s
test (1.10, long, ubuntu-latest)	49m 21s
test (1.11-nightly, short, ubuntu-latest)	1h 4m 46s
test (1.11-nightly, long, ubuntu-latest)	51m 17s
test (nightly, short, ubuntu-latest)	4m 27s
test (nightly, long, ubuntu-latest)	4m 17s
test (1.10, short, ubuntu-latest, depwarn=error)	52m 47s
test (1.10, long, ubuntu-latest, depwarn=error)	47m 22s
test (1.10, short, macOS, RPTU)	11m 35s
test (1.10, long, macOS, RPTU)	12m 28s
test (1.10, book, ubuntu-latest)	28m 14s
	9h 54m 54s

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