Migrate to syn3

[adamchalmers]

we use the Syn crate for writing a proc macro. Syn has released a new version, v3, which we should migrate to.

[codecov]

Codecov Report

❌ Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@c06523d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
modeling-session/src/lib.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1332   +/-   ##
=======================================
  Coverage        ?   33.62%           
=======================================
  Files           ?       36           
  Lines           ?     1805           
  Branches        ?        0           
=======================================
  Hits            ?      607           
  Misses          ?     1198           
  Partials        ?        0           

Flag	Coverage Δ
unittests	33.62% <30.76%> (?)

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