Update syn to v2 (#1064) #684

Job	Run time
lint	1m 36s
docs	2m 40s
coverage	3m 11s
Rust unit tests (ubuntu-latest)	3m 7s
Rust unit tests (macos-latest)	2m 58s
Rustfmt	7s
Rust unit tests (windows-latest)	5m 34s
test (macos-latest, 3.8)	13m 53s
typecheck	1m 32s
test (macos-latest, 3.9)	5m 33s
test (macos-latest, 3.10)	6m 57s
test (macos-latest, 3.11)	6m 3s
test (macos-latest, 3.12-dev)	6m 39s
test (ubuntu-latest, 3.8)	1m 35s
test (ubuntu-latest, 3.9)	3m 24s
test (ubuntu-latest, 3.10)	3m 36s
test (ubuntu-latest, 3.11)	3m 38s
test (ubuntu-latest, 3.12-dev)	5m 6s
test (windows-latest, 3.8)	3m 48s
test (windows-latest, 3.9)	6m 51s
test (windows-latest, 3.10)	4m 29s
test (windows-latest, 3.11)	5m 39s
test (windows-latest, 3.12-dev)	5m 32s
	1h 43m 28s

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