Bump and group dependencies

[fjarri]

• Bump rmp-serde to 1, base64 to 0.21, and pyo3 to 0.17.
• Remove second-level dependencies where applicable (e.g. use generic_array::typenum instead of typenum)
• Don't use getrandom/js unless building WASM bindings

[codecov-commenter]

Codecov Report

Merging #114 (f84d136) into master (50a7a6e) will decrease coverage by 0.09%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage   47.49%   47.39%   -0.10%     
==========================================
  Files          17       17              
  Lines        1017     1019       +2     
==========================================
  Hits          483      483              
- Misses        534      536       +2     

Impacted Files	Coverage Δ
umbral-pre/src/curve.rs	92.20% <ø> (ø)
umbral-pre/src/dem.rs	78.37% <ø> (ø)
umbral-pre/src/hashing.rs	100.00% <ø> (ø)
umbral-pre/src/key_frag.rs	84.94% <ø> (ø)
umbral-pre/src/keys.rs	66.66% <ø> (ø)
umbral-pre/src/serde_bytes.rs	40.00% <0.00%> (-1.87%)	⬇️

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