Fix duplicate schema set definitions for schemars 0.8, 0.9, and 1.0

[jonasbb]

The schema should be identical to BTreeSet.
Add schema implementations for other set types like HashSet and
IndexSet.
Add tests for SetLastValueWins, SetPreventDuplicates, MapFirstKeyWins,
MapPreventDuplicates, and JsonString.

Closes #906

[codecov]

Codecov Report

❌ Patch coverage is 91.37931% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.11%. Comparing base (066b9d4) to head (0650180).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
serde_with/src/schemars_1.rs	73.68%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #907      +/-   ##
==========================================
+ Coverage   67.80%   68.11%   +0.30%     
==========================================
  Files          36       36              
  Lines        9537     9537              
==========================================
+ Hits         6467     6496      +29     
+ Misses       3070     3041      -29     

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