Bindings simplification

[fjarri]

• Replaced AsBackend/FromBackend, .inner(), .new(), and pub backend with derived AsRef/From/Into where appropriate
• Use js_sys::Error instead of just JsValue in results
• Use a workaround with wasm-bindgen-derive to support Option<&T> (Simplify verification of KeyFrag in WASM bindings #25), &Vec<T> arguments (Support taking an array of cfrags in decrypt_reencrypted() in WASM bindings #23), and Vec<T> return values (Avoid converting the return value to JsValue in generate_kfrags() #26). Generate correct TypeScript signatures in this case.
• Removed serde usage in WASM bindings - nucypher-core will not need it (thanks to the above workaround)
• Added a workaround for Get rid of a getter for ciphertext in EncryptionResult #24 - now encrypt() returns an actual tuple in the WASM bindings

[codecov-commenter]

Codecov Report

Merging #103 (7196ed6) into master (0272fc5) will increase coverage by 1.79%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   58.76%   60.55%   +1.79%     
==========================================
  Files          17       17              
  Lines        1484     1440      -44     
==========================================
  Hits          872      872              
+ Misses        612      568      -44     

Impacted Files	Coverage Δ
umbral-pre/src/bindings_python.rs	0.00% <0.00%> (ø)
umbral-pre/src/bindings_wasm.rs	0.00% <0.00%> (ø)
umbral-pre/src/capsule_frag.rs	88.19% <ø> (ø)
umbral-pre/src/key_frag.rs	90.05% <ø> (ø)

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