Experimenting with by-name implicit parameters for generic derivation

[travisbrown]

This PR introduces a 2.13-only rewrite of circe-generic (as a new circe-generic-simple module, for now at least) that passes the same tests as circe-generic but completely removes DerivationMacros; the only macros are for the export mechanism and the @JsonCodec macro annotation. All of the shapeless.Lazy occurrences are gone—most have been replaced by by-name implicit parameters, but a few don't seem to be needed at all with 2.13.

In addition to having a less terrifying implementation, circe-generic-simple also provides much faster compilation (up to ~30% for some of my examples).

[travisbrown]

Note that we'll need scalameta/scalameta#1880 for Scalafmt to work here (it seems like you can set runner.dialect = dotty as a workaround).

[codecov-io]

Codecov Report

Merging #1180 into master will increase coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1180      +/-   ##
==========================================
+ Coverage   85.58%   85.89%   +0.31%     
==========================================
  Files          89       89              
  Lines        2885     2885              
  Branches      115      119       +4     
==========================================
+ Hits         2469     2478       +9     
+ Misses        416      407       -9

Impacted Files	Coverage Δ
...re/shared/src/main/scala/io/circe/KeyDecoder.scala	97.29% <0%> (-2.71%)	⬇️
...rc/main/scala/io/circe/numbers/BiggerDecimal.scala	90.05% <0%> (+2.92%)	⬆️
...re/shared/src/main/scala/io/circe/JsonNumber.scala	93.02% <0%> (+4.65%)	⬆️
...d/src/main/scala/io/circe/NonEmptySeqDecoder.scala	100% <0%> (+8.33%)	⬆️

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[johnynek]

Do you not need to cache codec in a lazy val to avoid executing it again and again for each call to apply?

[travisbrown]

Yeah, I meant to go back and do a pass for runtime performance stuff like this after getting it correct. I've just opened #1183. Thanks for catching.