Add schema composition keywords

[crmne]

Closes #37

Summary

• Add all_of support for Draft 2020-12 allOf
• Add none_of support for JSON Schema not
• Wrap multiple none_of child schemas in not: { anyOf: [...] }
• Cover direct and array item composition specs

Verification

• bundle exec rake

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.89%. Comparing base (7ab8ada) to head (3eccd8c).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           issue-34-const-validation-keyword      #48      +/-   ##
=====================================================================
+ Coverage                              94.75%   94.89%   +0.14%     
=====================================================================
  Files                                     12       12              
  Lines                                    381      392      +11     
  Branches                                  75       76       +1     
=====================================================================
+ Hits                                     361      372      +11     
  Misses                                    20       20              

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