Unpin serde-json and remove json_internal imports

[Swatinem]

Also adds a rust-toolchain.toml file so that the rustup wrapper will automatically pick this up, and commits the Cargo.lock file as the conventions regarding that were changed in https://blog.rust-lang.org/2023/08/29/committing-lockfiles.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.12%. Comparing base (d986d4d) to head (f6d7c80).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   98.12%   98.12%   -0.01%     
==========================================
  Files          17       17              
  Lines        6248     6225      -23     
==========================================
- Hits         6131     6108      -23     
  Misses        117      117              

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[matt-codecov]

:o wonder why i pinned it. i feel like it was pinned on the crates.io page when i added it

in any case, thanks for these hygiene changes!