Add serde support to linfa-preprocessing and linfa-bayes

[YuhanLiin]

Addresses #291

[codecov-commenter]

Codecov Report

Base: 38.50% // Head: 38.41% // Decreases project coverage by -0.09% ⚠️

Coverage data is based on head (7695ef7) compared to base (4855360).
Patch coverage: 80.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
- Coverage   38.50%   38.41%   -0.09%     
==========================================
  Files          94       94              
  Lines        6220     6221       +1     
==========================================
- Hits         2395     2390       -5     
- Misses       3825     3831       +6     

Impacted Files	Coverage Δ
algorithms/linfa-bayes/src/gaussian_nb.rs	47.29% <ø> (ø)
algorithms/linfa-bayes/src/hyperparams.rs	10.52% <ø> (ø)
algorithms/linfa-bayes/src/multinomial_nb.rs	46.55% <ø> (ø)
...gorithms/linfa-preprocessing/src/linear_scaling.rs	47.61% <ø> (ø)
algorithms/linfa-preprocessing/src/norm_scaling.rs	54.54% <ø> (-13.64%)	⬇️
...ms/linfa-preprocessing/src/tf_idf_vectorization.rs	36.11% <ø> (ø)
algorithms/linfa-preprocessing/src/whitening.rs	55.76% <ø> (ø)
.../linfa-preprocessing/src/countgrams/hyperparams.rs	64.10% <66.66%> (-1.69%)	⬇️
...gorithms/linfa-preprocessing/src/countgrams/mod.rs	68.90% <100.00%> (ø)
... and 4 more

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