Better singleton types support

[MateuszKubuszok]

TODO:

• utilities for literal singletons (Boolean, Int, Long, Float, Double, Char, String)
• Singleton datatype aggregating: literal singletons, (), null, .type and objects
• rule transforming into Singleton type (excluding None and Unit, with explanation)
• fallback on Singleton type (instead of just Unit, excluding None - document why)
• tests
  • fill Null (all)
  • fill literal-based singleton types (2.13/3)
  • fill case object (all)
  • fill Enum.Value.type (3)
  • into Null (all)
  • into literal-based singleton types (2.13/3)
  • into case object (all)
  • into Enum.Value.type (3)
• docs

[codecov]

Codecov Report

Attention: Patch coverage is 87.30159% with 8 lines in your changes missing coverage. Please review.

Project coverage is 87.77%. Comparing base (ae098a6) to head (da2ed8f).
Report is 89 commits behind head on master.

Files with missing lines	Patch %	Lines
...nternal/compiletime/datatypes/SingletonTypes.scala	86.95%	3 Missing ⚠️
...d/chimney/internal/compiletime/TypesPlatform.scala	84.61%	2 Missing ⚠️
.../chimney/integrations/PartiallyBuildIterable.scala	0.00%	2 Missing ⚠️
...scalaland/chimney/internal/compiletime/Types.scala	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #559      +/-   ##
==========================================
+ Coverage   87.57%   87.77%   +0.19%     
==========================================
  Files         149      151       +2     
  Lines        5845     5899      +54     
  Branches      489      494       +5     
==========================================
+ Hits         5119     5178      +59     
+ Misses        726      721       -5     

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