Define json type to string trait

[macisamuele]

The goal of this PR is to ensure that :

• JsonType instances are not forced to be Debug types
• JsonType instances are ensured to be string-representable via a new trait (JsonTypeToString)

NOTE: JsonTypeToString does provide a default implementation but different trait implementations might override its implementation to use more performant solutions

[codecov]

Codecov Report

Merging #41 into master will increase coverage by 2.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
+ Coverage   94.23%   96.40%   +2.17%     
==========================================
  Files           9        9              
  Lines        1300     1337      +37     
==========================================
+ Hits         1225     1289      +64     
+ Misses         75       48      -27     

Impacted Files	Coverage Δ
src/json_type.rs	96.45% <100.00%> (+11.67%)	⬆️
src/rust_type_impl.rs	91.50% <100.00%> (+6.08%)	⬆️
src/traits/_json.rs	98.72% <100.00%> (+0.03%)	⬆️
src/traits/_pyo3.rs	96.78% <100.00%> (+0.10%)	⬆️
src/traits/_serde_json.rs	97.41% <100.00%> (+0.09%)	⬆️
src/traits/_serde_yaml.rs	97.37% <100.00%> (+0.05%)	⬆️

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