Serde serialization rework

[fjarri]

This is the foundation needed for nucypher/nucypher-core#17

• Add 0x to hex-encoded bytestrings.
• Expose the bytes/hex/base64 serialization machinery (as serde_bytes module) so that nucypher-core could use it instead of serde_bytes crate and its own arrays_as_bytes.

[codecov-commenter]

Codecov Report

Merging #94 (b10de46) into master (8138b5b) will decrease coverage by 0.33%.
The diff coverage is 75.30%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   60.31%   59.98%   -0.34%     
==========================================
  Files          17       17              
  Lines        1449     1472      +23     
==========================================
+ Hits          874      883       +9     
- Misses        575      589      +14     

Impacted Files	Coverage Δ
umbral-pre/src/traits.rs	76.66% <ø> (ø)
umbral-pre/src/serde_bytes.rs	69.69% <69.69%> (ø)
umbral-pre/src/capsule.rs	91.26% <100.00%> (ø)
umbral-pre/src/capsule_frag.rs	88.19% <100.00%> (ø)
umbral-pre/src/key_frag.rs	90.05% <100.00%> (+0.11%)	⬆️
umbral-pre/src/keys.rs	77.77% <100.00%> (ø)

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[piotr-roslaniec]

LGTM 🎉

Would you say it mandates a major version bump?