msgs: make TlsListElement/ListLength pub(crate).

[cpu]

This commit resolves two TODOs left in msgs/codec.rs about using pub(crate) visibility for TlsListElement and ListLength once MSRV allows it. That time has come :)

[codecov]

Codecov Report

Merging #1379 (2dbe6c3) into main (d453575) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1379   +/-   ##
=======================================
  Coverage   96.35%   96.35%           
=======================================
  Files          62       62           
  Lines       14580    14580           
=======================================
  Hits        14049    14049           
  Misses        531      531           

Files Changed	Coverage Δ
rustls/src/msgs/codec.rs	98.59% <ø> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[cpu]

rustls / Features (pull_request) Failing after 2m

Another network flake pulling dependencies.

[ctz]

rustls / Features (pull_request) Failing after 2m

Another network flake pulling dependencies.

Highly suspicious that it always seems to be the same dependency 🤨

error: failed to get `syn` as a dependency of package `wasm-bindgen-macro-support v0.2.87`
    ... which satisfies dependency `wasm-bindgen-macro-support = "=0.2.87"` of package `wasm-bindgen-macro v0.2.87`
    ... which satisfies dependency `wasm-bindgen-macro = "=0.2.87"` of package `wasm-bindgen v0.2.87`
    ... which satisfies dependency `wasm-bindgen = "^0.2.87"` of package `js-sys v0.3.64`
    ... which satisfies dependency `js-sys = "^0.3.64"` of package `web-sys v0.3.64`
    ... which satisfies dependency `web-sys = "^0.3.37"` of package `ring v0.[16](https://github.com/rustls/rustls/actions/runs/5727745887/job/15520845697#step:4:17).20`
    ... which satisfies dependency `ring = "^0.16.20"` of package `rustls v0.22.0-alpha.0 (/home/runner/work/rustls/rustls/rustls)`
    ... which satisfies path dependency `rustls` of package `rustls-examples v0.0.1 (/home/runner/work/rustls/rustls/examples)