Better singleton types support

[MateuszKubuszok]

TODO:

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

[codecov[bot]]

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 :warning:
...d/chimney/internal/compiletime/TypesPlatform.scala	84.61%	2 Missing :warning:
.../chimney/integrations/PartiallyBuildIterable.scala	0.00%	2 Missing :warning:
...scalaland/chimney/internal/compiletime/Types.scala	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.