Implement serde for rpds types, submission 2

[idupree]

This patch set adds serde as an optional dependency; serialization/deserialization code isn't compiled if it's not used. Addresses issue #1. Improved version of pull request #2.

You can run tests that include serialization/deserialization code using

cargo test --features serde

or more likely if you want the tests to finish somewhat promptly,

cargo test --release --features serde

[codecov-io]

Codecov Report

Merging #3 into master will increase coverage by 0.91%. The diff coverage is 98.03%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
+ Coverage   93.84%   94.76%   +0.91%     
==========================================
  Files          11       11              
  Lines        1949     2042      +93     
==========================================
+ Hits         1829     1935     +106     
+ Misses        120      107      -13

Impacted Files	Coverage Δ
src/lib.rs	100% <ø> (ø)	:arrow_up:
src/map/red_black_tree_map/mod.rs	93.42% <100%> (+1.4%)	:arrow_up:
src/set/hash_trie_set/mod.rs	95.94% <100%> (+0.86%)	:arrow_up:
src/stack/mod.rs	97.1% <100%> (+0.38%)	:arrow_up:
src/sequence/vector/mod.rs	95.95% <100%> (+0.96%)	:arrow_up:
src/map/hash_trie_map/mod.rs	96.48% <100%> (+0.59%)	:arrow_up:
src/set/red_black_tree_set/mod.rs	96.05% <100%> (+0.81%)	:arrow_up:
src/queue/mod.rs	90% <90%> (+0.83%)	:arrow_up:
src/sequence/list/mod.rs	93.19% <94.73%> (+0.28%)	:arrow_up:
... and 4 more

---

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 7382032...7391d9a. Read the comment docs.

[orium]

Merged #4