Add a slim builder that doesn't call to_string right away

[sunsided]

This addresses the ideas behind #6. It uses a recursive type construction outlined in this gist and can deal with any type (and type reference).

[codecov]

Codecov Report

Attention: Patch coverage is 87.98283% with 28 lines in your changes missing coverage. Please review.

Project coverage is 92.62%. Comparing base (873c41d) to head (9fe6196).

❗ Current head 9fe6196 differs from pull request most recent head d58a2e3

Please upload reports for the commit d58a2e3 to get more accurate results.

Files	Patch %	Lines
src/slim.rs	87.21%	28 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main      #12      +/-   ##
===========================================
- Coverage   100.00%   92.62%   -7.38%     
===========================================
  Files            1        2       +1     
  Lines          171      393     +222     
===========================================
+ Hits           171      364     +193     
- Misses           0       29      +29     

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