Add Fressian Serde via clojure.data.fressian

[bobby]

Checklist

• [x] tests
• [x] updated CHANGELOG (the "unreleased" section)

[bobby]

Thanks for approving, I look forward to a release with this added Serde!

[creese]

@bobby Please sign off on the commits. If you want to rebase these down to one commit, that's fine.

Instructions on how to sign off: https://github.com/FundingCircle/jackdaw/blob/master/CONTRIBUTING.md#signoff-on-commits

[codecov[bot]]

Codecov Report

Merging #209 into master will increase coverage by 0.06%. The diff coverage is 86.36%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   79.31%   79.37%   +0.06%     
==========================================
  Files          41       42       +1     
  Lines        2548     2570      +22     
  Branches      150      150              
==========================================
+ Hits         2021     2040      +19     
- Misses        377      380       +3     
  Partials      150      150

Impacted Files	Coverage Δ
src/jackdaw/serdes.clj	71.42% <50%> (-8.58%)	:arrow_down:
src/jackdaw/serdes/fressian.clj	90% <90%> (ø)

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[bobby]

@creese I've signed off on the commits, and the checks have passed again.