Rework internal TokenStream representation #7586

Job	Run time
doc tests	3m 5s
build test linux x86-64	3m 55s
check-rust	3m 54s
build test windows x86-64	10m 29s
check-website	9s
scarb-metadata compatibility check	40s
test linux x86-64 1/8	17m 10s
test linux x86-64 2/8	18m 46s
test linux x86-64 3/8	15m 20s
test linux x86-64 4/8	14m 19s
test linux x86-64 5/8	12m 22s
test linux x86-64 6/8	11m 6s
test linux x86-64 7/8	10m 35s
test linux x86-64 8/8	11m 5s
test windows x86-64 1/8	15m 23s
test windows x86-64 2/8	17m 7s
test windows x86-64 3/8	15m 4s
test windows x86-64 4/8	13m 12s
test windows x86-64 5/8	12m 33s
test windows x86-64 6/8	9m 58s
test windows x86-64 7/8	10m 6s
test windows x86-64 8/8	10m 54s
	3h 57m 12s

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