Error handling for Rust and Python

[fjarri]

Based on top of #41
Fixes #20
Fixes #31

• returning Result instead of Option in Rust methods
• raising corresponding Python exceptions in Python bindings
• implemented __eq__(), __hash__() and __repr__() too where appropriate (using the new HasSerializableBackend helper)
• update Python and Rust docs, and Python type annotations

[codecov-io]

Codecov Report

Merging #45 (aac9775) into master (128eb0c) will increase coverage by 0.33%.
The diff coverage is 93.10%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   94.64%   94.98%   +0.33%     
==========================================
  Files          10       11       +1     
  Lines         766      777      +11     
==========================================
+ Hits          725      738      +13     
+ Misses         41       39       -2     

Impacted Files	Coverage Δ
umbral-pre/src/dem.rs	92.50% <85.00%> (-1.95%)	⬇️
umbral-pre/src/key_frag.rs	89.24% <86.66%> (+2.84%)	⬆️
umbral-pre/src/capsule_frag.rs	95.08% <88.00%> (+2.32%)	⬆️
umbral-pre/src/traits.rs	96.42% <88.88%> (-3.58%)	⬇️
umbral-pre/src/curve.rs	92.00% <94.11%> (-3.51%)	⬇️
umbral-pre/src/keys.rs	95.34% <95.34%> (ø)
umbral-pre/src/capsule.rs	99.03% <96.77%> (-0.97%)	⬇️
umbral-pre/src/hashing.rs	97.87% <100.00%> (-0.31%)	⬇️
umbral-pre/src/hashing_ds.rs	100.00% <100.00%> (ø)
umbral-pre/src/params.rs	100.00% <100.00%> (ø)
... and 2 more

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[piotr-roslaniec]

LGTM 👍