Reduce Serialized Size of HybridInfo

[tyurek]

This PR does a few things

1. Changes the codebase to assume helper_origin is a static string and does not need to be stored anywhere. It is removed from structs and outputs and simply grabbed from the place where it's defined if needed.
2. Additionally removes DOMAIN from serialized HybridInfo bytes.
3. Adds a second "to_bytes" method for HybridInfo structs. While we don't need hybrid_info and DOMAIN to deserialize HybridInfo, they are needed when we're actually encrypting/decrypting. For those purposes, a new to_enc_bytes() function is added. (Let me know if anyone can come up with a better name for this).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.28%. Comparing base (b27a4b3) to head (1d01a77).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1485      +/-   ##
==========================================
+ Coverage   93.26%   93.28%   +0.02%     
==========================================
  Files         239      239              
  Lines       43675    43532     -143     
==========================================
- Hits        40734    40610     -124     
+ Misses       2941     2922      -19     

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