Serialization methods for Eth contracts

[fjarri]

• Added Signature::to_be_bytes(), Capsule::to_bytes_simple(), and CapsuleFrag::to_bytes_simple() to use in Ethereum contracts.

Also:

• Added the missing VerifiedCapsuleFrag::unverify() method in WASM bindings.
• Added the missing Signature::to_der_bytes() and from_der_bytes() that were missing in the Rust library but present in the bindings.

[codecov-commenter]

Codecov Report

Merging #115 (4292071) into master (1b24c75) will decrease coverage by 1.65%.
The diff coverage is 23.25%.

❗ Current head 4292071 differs from pull request most recent head 9569a34. Consider uploading reports for the commit 9569a34 to get more accurate results

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   47.39%   45.75%   -1.65%     
==========================================
  Files          17       17              
  Lines        1019     1049      +30     
==========================================
- Hits          483      480       -3     
- Misses        536      569      +33     

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	65.38% <0.00%> (-14.31%)	⬇️
umbral-pre/src/keys.rs	65.38% <66.66%> (-1.29%)	⬇️
umbral-pre/src/capsule.rs	87.80% <100.00%> (-0.44%)	⬇️
umbral-pre/src/pre.rs	85.29% <100.00%> (ø)

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