[native] Return tuples instead of lists in CST nodes

[zsol]

Summary

This pr convers Vec<> to tuple during into_py, which reduces the memory usage of the CST on the Python side by about 10%.

Test Plan

CI

[codecov-commenter]

Codecov Report

Merging #631 (423a52d) into main (595d8c3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #631   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files         245      245           
  Lines       25281    25281           
=======================================
  Hits        23963    23963           
  Misses       1318     1318           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 595d8c3...423a52d. Read the comment docs.