Avoid stringly-typed APIs in several spots

[notriddle]

From the "let the knife do the work" school of Rust, this pull request has a net reduction of about twenty lines by avoiding manual stringification in favour of implementing the serde traits directly on the corresponding API types.

For more information on some of what I'm doing here, read https://serde.rs/container-attrs.html.

Also fixes #49; instead of implementing Display, it implements Serialize.

[codecov-io]

Codecov Report

Merging #74 into master will increase coverage by 0.27%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
+ Coverage    50.5%   50.77%   +0.27%     
==========================================
  Files          24       24              
  Lines        1689     1668      -21     
==========================================
- Hits          853      847       -6     
+ Misses        836      821      -15

Impacted Files	Coverage Δ
src/core/receive.rs	91.11% <ø> (ø)	⬆️
src/core/send.rs	82.35% <ø> (ø)	⬆️
src/core/mailbox.rs	52.88% <ø> (+0.5%)	⬆️
src/core/api.rs	28.57% <ø> (-3.51%)	⬇️
src/core.rs	69.11% <100%> (-1.72%)	⬇️
src/core/timing.rs	90.47% <100%> (ø)	⬆️
src/core/order.rs	89.28% <100%> (+3.07%)	⬆️
src/core/server_messages.rs	100% <100%> (ø)	⬆️
src/core/rendezvous.rs	52.04% <16.66%> (+0.3%)	⬆️
src/core/boss.rs	26.41% <50%> (-0.3%)	⬇️
... and 8 more

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[warner]

thanks! that's awesome, I learn more about Rust every day :)