feat: Dont make copies of structs for validation

[asmfreak]

This PR fixes a small problem with validation implementation:
Current implementation uses dst.Interface() which returns a value(T), not a pointer(*T) of a struct T. This can be a performance hit on bigger structures as compiler would have to partially copy the struct T. And since validation is performed on each structure, the whole thing would be copied at least N times (where N is the structure depth).

Passing by value also does not play nicely with other (non-reflection based) validation libraries, such as github.com/go-ozzo/ozzo-validation, since validating a structure there can require a method call, which is not possible. Converting an interface{} with value is not an option also - reflect.Value.Addr() is prohibited for Values, created via reflect.ValueOf(v).

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.59%. Comparing base (abc7083) to head (c510242).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #665      +/-   ##
==========================================
- Coverage   77.61%   77.59%   -0.03%     
==========================================
  Files          22       22              
  Lines        7953     7959       +6     
==========================================
+ Hits         6173     6176       +3     
- Misses       1362     1364       +2     
- Partials      418      419       +1     

[asmfreak]

I'm not sure, if the if branch is needed, really. Please advise, I'll modify the code