Trait implementations and error handling

[fjarri]

Based on top of #50

• Move HasName trait from Python bindings into the main library and rename it to HasTypeName
• Add a Display implementation for public types in the main library and use it in Python/WASM bindings (was formerly only implemented in Python bindings)
• Remove GenericError from Python bindings; in the cases where it is used it can be replaced by ValueError
• from_array() can only return a ConstructionError now, and not a size mismatch error (because the array size is already statically asserted by the compiler)
• Add toBytes()/fromBytes()/toString() methods to public objects in WASM bindings. Fixes Expose serialization in bindings #11 (if we decide to use some third-party serialization, like protobuf, it should be made a separate issue)
• Merge DeserializationError::TooManyBytes/TooFewBytes into a single SizeMismatch
• Retain more information in deserialization errors: the error message and the problem type for construction errors (helps when deserializing structures with nested elements), the expected/received bytestring size for size mismatch
• Raise exceptions properly instead of using unwrap() in WASM bindings. Fixes Propagate errors without panics when possible in WASM bindings #14, fixes Only enable console_error_panic_hook in tests #16 (console_error_panic_hook is not needed anymore since there are no panics)
• Move error messages from Python bindings into Display implementations of error types in the main library, and use them from Python/WASM bindings to form exceptions
• Expose SecretKeyFactory in WASM bindings
• Use camel case for method names in WASM bindings

[codecov-commenter]

Codecov Report

Merging #51 (1b57bdd) into master (8a7f9c7) will decrease coverage by 10.51%.
The diff coverage is 25.36%.

@@             Coverage Diff             @@
##           master      #51       +/-   ##
===========================================
- Coverage   94.28%   83.77%   -10.52%     
===========================================
  Files          11       11               
  Lines         805      912      +107     
===========================================
+ Hits          759      764        +5     
- Misses         46      148      +102     

Impacted Files	Coverage Δ
umbral-pre/src/dem.rs	77.08% <0.00%> (-15.42%)	⬇️
umbral-pre/src/pre.rs	93.33% <0.00%> (-6.67%)	⬇️
umbral-pre/src/capsule.rs	88.98% <7.69%> (-10.09%)	⬇️
umbral-pre/src/capsule_frag.rs	86.02% <14.28%> (-8.33%)	⬇️
umbral-pre/src/keys.rs	74.16% <15.62%> (-21.71%)	⬇️
umbral-pre/src/key_frag.rs	81.05% <17.64%> (-6.45%)	⬇️
umbral-pre/src/curve.rs	83.92% <33.33%> (-10.19%)	⬇️
umbral-pre/src/traits.rs	76.13% <51.21%> (-20.42%)	⬇️

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