Add support for Scala 2.13

[vlovgr]

Adds support for Scala 2.13 to all modules but ciris-squants (see typelevel/squants#344).

While adding support for Scala 2.13, the following changes have been made.

• Drop support for Scala 2.11.
• Drop support for Scala.js and Scala Native.
  • Looking at download figures, there's not much interest in Ciris for these platforms.
  • The build.sbt build configuration becomes significantly easier to manage.
  • No need for platform-specific sources, makes code easier to manage.
• Remove the 'Creating new modules' section from the contributing guide.
  • It needs re-writing after significant changes to build.sbt.
• Tests have been split from the tests project into individual modules.
  • Squants is still not available for Scala 2.13, so prevents tests from being run.
  • Splitting the tests allows us to run them on 2.13 for modules with 2.13 support.

Fixes #220, replaces #243.

[codecov]

Codecov Report

Merging #244 into master will decrease coverage by 1.24%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   99.09%   97.85%   -1.25%     
==========================================
  Files          47       44       -3     
  Lines         885      885              
  Branches        7       12       +5     
==========================================
- Hits          877      866      -11     
- Misses          8       19      +11

Impacted Files	Coverage Δ
...main/scala/ciris/decoders/MathConfigDecoders.scala	100% <ø> (ø)
...ala/ciris/decoders/JavaNioFileConfigDecoders.scala	100% <ø> (ø)
...dules/core/src/main/scala/ciris/ConfigErrors.scala	100% <ø> (ø)
...odules/core/src/main/scala/ciris/ConfigError.scala	100% <ø> (ø)
...n/scala/ciris/decoders/JavaNetConfigDecoders.scala	100% <ø> (ø)
...ules/core/src/main/scala/ciris/ConfigResults.scala	100% <ø> (ø)
.../scala/ciris/decoders/JavaUtilConfigDecoders.scala	100% <ø> (ø)
...ciris/generic/decoders/GenericConfigDecoders.scala	100% <ø> (ø)
...rc/main/scala/ciris/api/syntax/FlatMapSyntax.scala	100% <ø> (ø)
...odules/core/src/main/scala/ciris/api/package.scala	100% <ø> (ø)
... and 36 more

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