dhkem-x25519: use curve utils in hpke/common.

[dajiaji]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.14%. Comparing base (15d4703) to head (1579838).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #603      +/-   ##
==========================================
+ Coverage   79.53%   83.14%   +3.61%     
==========================================
  Files          77       73       -4     
  Lines        6786     6415     -371     
  Branches      609      590      -19     
==========================================
- Hits         5397     5334      -63     
+ Misses       1378     1072     -306     
+ Partials       11        9       -2     

Flag	Coverage Δ
unittests	83.14% <100.00%> (+3.61%)	⬆️

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