Serialize AssetId by identity.

[davidmorgan]

For #3811.

Serialize AssetId by identity, with a separate list of values.

Remove some unused/old code that was the old way of doing something similar.

This significantly reduces the graph size, but is still not the final state: next step after this is to keep input sets as trees, and serialize those as trees and by identity.

Benchmarks

Before this PR

with this PR
json_serializable
shape,libraries,clean/ms,no changes/ms,incremental/ms,json/KiB
loop,1,23492,3887,5292,417
loop,100,23786,3900,6928,2564
loop,250,27987,4494,9667,12674
loop,500,32715,5789,16602,47884
loop,750,37391,8292,22735,106042
loop,1000,43723,11432,30665,191073
forwards,1,22246,3762,5264,417
forwards,100,24092,3960,7265,1741
forwards,250,27831,4231,9800,7467
forwards,500,29865,5024,14991,26973
forwards,750,36458,6234,20061,58929
forwards,1000,39756,8070,28756,105307
backwards,1,22162,3845,5364,417
backwards,100,24036,4054,7900,1762
backwards,250,27503,4349,10938,7594
backwards,500,31306,5024,15695,27471
backwards,750,37975,6045,22651,60042
backwards,1000,43777,8021,27442,107280

after

json_serializable
shape,libraries,clean/ms,no changes/ms,incremental/ms,json/KiB
loop,1,23153,3936,5187,339
loop,100,24380,4026,7118,702
loop,250,26753,4199,10192,1982
loop,500,32433,4526,14631,6068
loop,750,35511,5059,19585,12595
loop,1000,41457,5664,26464,21574
forwards,1,23234,3912,5161,339
forwards,100,25944,4070,7481,609
forwards,250,26586,3938,9369,1383
forwards,500,32053,4194,14056,3649
forwards,750,35001,4608,18724,7135
forwards,1000,40243,5003,23503,11853
backwards,1,23745,3929,5077,339
backwards,100,25092,3977,7655,605
backwards,250,27593,4057,10292,1373
backwards,500,33167,4189,15592,3628
backwards,750,38389,4721,19006,7105
backwards,1000,43074,5071,25351,11810

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