Rename TestingType into RustType

[macisamuele]

This PR renames json_trait_rs::TestingType into json_trait_rs::RustType.
This is done because external libraries might take advantage of it and it would be "not nice" to force them to have to use an additional feature.

NOTE: As TestingType or RustType depends on rust specific information (no other dependencies) it would not add much binary code in the final artifact.

[codecov]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is 91.66%.

@@           Coverage Diff           @@
##           master       #3   +/-   ##
=======================================
  Coverage   93.48%   93.48%           
=======================================
  Files           8        8           
  Lines         537      537           
=======================================
  Hits          502      502           
  Misses         35       35

Impacted Files	Coverage Δ
src/lib.rs	0% <ø> (ø)	⬆️
src/json_type.rs	89.47% <0%> (ø)	⬆️
src/index.rs	92.68% <100%> (ø)	⬆️
src/traits/_serde_json.rs	95.18% <100%> (ø)	⬆️
src/traits/_serde_yaml.rs	95.18% <100%> (ø)	⬆️
src/fragment.rs	100% <100%> (ø)	⬆️
src/traits/_json.rs	97.75% <100%> (ø)	⬆️
src/rust_type.rs	91.97% <93.33%> (ø)

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