Add json size to benchmark.

[davidmorgan]

For #3811.

So we can see the coming improvements :)

Current output ... yes, the biggest one is 420MB.

json_serializable
shape,libraries,clean/ms,no changes/ms,incremental/ms,json/KiB
loop,1,23898,3880,5264,506
loop,100,25214,4205,7814,5100
loop,250,28638,5057,11263,27233
loop,500,44939,7361,18271,104746
loop,750,81607,12244,26289,233040
loop,1000,142847,18559,32439,419976
forwards,1,22143,3788,5312,507
forwards,100,24645,4010,7375,3267
forwards,250,28664,4691,10824,15619
forwards,500,36589,6209,16613,58082
forwards,750,48166,8959,24655,127888
forwards,1000,78474,11346,28550,228996
backwards,1,21719,3837,5240,507
backwards,100,25152,4139,7780,3312
backwards,250,28480,4413,12578,15877
backwards,500,36281,6406,18998,59086
backwards,750,57325,8617,26358,130126
backwards,1000,82449,11933,30403,232957

[github-actions]

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