Simplify construction of AAD and Nonce values.

[briansmith]

No description provided.

[coveralls]

Coverage decreased (-0.01%) to 96.512% when pulling 35f3615 on briansmith:b/simplify-aad into 666a2cb on ctz:master.

[codecov-io]

Codecov Report

Merging #278 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
- Coverage   96.52%   96.51%   -0.02%     
==========================================
  Files          50       50              
  Lines        8318     8286      -32     
==========================================
- Hits         8029     7997      -32     
  Misses        289      289

Impacted Files	Coverage Δ
src/key_schedule.rs	97.6% <100%> (-0.04%)	⬇️
src/cipher.rs	96.38% <100%> (-0.38%)	⬇️

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[briansmith]

Note: Instead of doing this nonce stuff, it would be straightforward to use OpeningKey and SealingKey with a custom NonceSequence implementation based on the make_tls13_nonce() I wrote here. However, my understanding is that you explicitly do prefer to use LessSafeKey instead, so I avoided including those changes here.

[ctz]

Thanks!