Add SecretKey::public_key() shortcut

[fjarri]

Paired with nucypher/pyUmbral#271

See the rationale there. Briefly, this is a more used way, and it coincides with RustCrypto API

[codecov-commenter]

Codecov Report

Merging #52 (78e55c5) into master (c9c5770) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #52   +/-   ##
=======================================
  Coverage   83.77%   83.77%           
=======================================
  Files          11       11           
  Lines         912      912           
=======================================
  Hits          764      764           
  Misses        148      148           

Impacted Files	Coverage Δ
umbral-pre/src/capsule.rs	88.98% <100.00%> (ø)
umbral-pre/src/capsule_frag.rs	86.02% <100.00%> (ø)
umbral-pre/src/key_frag.rs	81.05% <100.00%> (ø)
umbral-pre/src/keys.rs	74.16% <100.00%> (ø)
umbral-pre/src/pre.rs	93.33% <100.00%> (ø)

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