Remove clone() calls from library internals

[fjarri]

Fixes #88

The following API changes were made:

• Signer::new() consumes the given SecretKey (fixes Should Signer consume SecretKey on creation? #61).
• KeyFrag::verify() and CapsuleFrag::verify() consume the given kfrag/cfrag.
• As a consequence, KeyFrag::verify() and CapsuleFrag::verify() return the original frag on error (as a tuple with the error itself), for logging purposes (since the original object is not available anymore).
• reencrypt() consumes the cfrag (but not the capsule, since it wasn't being cloned inside).
• VerifiedKeyFrag::to_unverified() and VerifiedCapsuleFrag::to_unverified() were renamed to unverify() and consume the corresponding frag.

[codecov-commenter]

Codecov Report

Merging #91 (ab74c02) into master (cd3266b) will decrease coverage by 0.36%.
The diff coverage is 64.17%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   68.70%   68.34%   -0.37%     
==========================================
  Files          16       16              
  Lines        1342     1333       -9     
==========================================
- Hits          922      911      -11     
- Misses        420      422       +2     

Impacted Files	Coverage Δ
umbral-pre/bench/bench.rs	0.00% <0.00%> (ø)
umbral-pre/src/bindings_python.rs	0.00% <0.00%> (ø)
umbral-pre/src/capsule_frag.rs	88.19% <69.23%> (-1.39%)	⬇️
umbral-pre/src/key_frag.rs	89.94% <86.66%> (-0.56%)	⬇️
umbral-pre/src/capsule.rs	91.26% <100.00%> (-0.21%)	⬇️
umbral-pre/src/keys.rs	77.77% <100.00%> (-0.46%)	⬇️
umbral-pre/src/pre.rs	92.06% <100.00%> (-0.25%)	⬇️

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