Gate serde support under a feature

[fjarri]

serde is a pretty fat crate and is not needed for JS/Python bindings (and possibly not needed by many users of the main crate either). So this PR hides it under an optional feature.

Also it adds a list of available feature to the docs and marks gated methods with a doc attribute.

[codecov-commenter]

Codecov Report

Merging #82 (71df239) into master (f29791c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   81.12%   81.12%           
=======================================
  Files          15       15           
  Lines        1118     1118           
=======================================
  Hits          907      907           
  Misses        211      211           

Impacted Files	Coverage Δ
umbral-pre/src/capsule.rs	90.76% <ø> (ø)
umbral-pre/src/capsule_frag.rs	90.07% <ø> (ø)
umbral-pre/src/key_frag.rs	90.96% <ø> (ø)
umbral-pre/src/keys.rs	77.77% <ø> (ø)
umbral-pre/src/pre.rs	92.18% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f29791c...71df239. Read the comment docs.