Establish an MSRV and add a CI test to ensure compatibility

[jmwample]

This PR tries to establish a ground level for the Minimum Supported Rust Version (MSRV).

This has required a few things.

1. Disable o5 crate in the workspace Cargo.toml
   • o5 requires the ml_kem package at the moment which forces an msrv of 1.74, but we can do better.
2. Test with the minimum version of ALL dependencies
   • This includes transitive dependencies
     • several of these actually allow dependencies of old enough version that it breaks things
     • to fix this I manually found the exact minimum acceptable version of those crates and added it to the Cargo.toml of the (internal) crate that uses them.
3. Downgrade some of the patterns that I am using to implement things that aren't supported in older versions of rustc.
   • For example a trait with a function whose return contains an impl

With all of this we are left with the msrv of 1.70 as the tor-* (tor-bytes, tor-cell, etc.) have an msrv of 1.70 and cannot be downgraded without allowing some critical vulnerability.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.22222% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 77.72%. Comparing base (02135f7) to head (546f93c).

Files	Patch %	Lines
crates/ptrs/src/helpers.rs	68.00%	8 Missing ⚠️
crates/obfs4/src/testing.rs	73.07%	0 Missing and 7 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   78.00%   77.72%   -0.28%     
==========================================
  Files          36       35       -1     
  Lines        5642     5617      -25     
  Branches     5642     5617      -25     
==========================================
- Hits         4401     4366      -35     
- Misses        852      860       +8     
- Partials      389      391       +2     

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