`StaticKeyPair` in agreement module

[richard-w]

Adds agreement::StaticKeyPair and agreement::agree_static

Also restructures some existing tests to avoid duplication of code and test vectors.

Closes: #331

[richard-w]

The test failure is a spurious error in appveyor.

[codecov[bot]]

Codecov Report

Merging #1081 (0da2432) into main (27200d4) will increase coverage by 0.03%. The diff coverage is 96.21%.

@@            Coverage Diff             @@
##             main    #1081      +/-   ##
==========================================
+ Coverage   91.97%   92.01%   +0.03%     
==========================================
  Files         106      106              
  Lines       14238    14395     +157     
==========================================
+ Hits        13096    13246     +150     
- Misses       1142     1149       +7     

Impacted Files	Coverage Δ
tests/agreement_tests.rs	95.51% <94.00%> (-1.80%)	:arrow_down:
src/agreement.rs	88.00% <98.82%> (+2.65%)	:arrow_up:
src/aead/aes.rs	75.23% <0.00%> (-0.48%)	:arrow_down:
src/ec/curve25519/x25519.rs	98.86% <0.00%> (+0.05%)	:arrow_up:
tests/aead_tests.rs	97.13% <0.00%> (+0.35%)	:arrow_up:
src/ec/keys.rs	98.21% <0.00%> (+1.78%)	:arrow_up:
src/hkdf.rs	88.50% <0.00%> (+4.81%)	:arrow_up:

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