Optimize the string allocations

[DamianEdwards]

The validator has to allocate strings for each field it validates, including adding a level prefix as it descends the object graph, even if there don't end up being any errors. Would be good to look at if there's a way to minimize these allocations.

Examples:

• MiniValidation/src/MiniValidation/MiniValidator.cs

  Line 185 in 7efcac2

  var thePrefix = $"{prefix}{propertyDetails.Name}";

• MiniValidation/src/MiniValidation/MiniValidator.cs

  Line 190 in 7efcac2

  var thePrefix = $"{prefix}{propertyDetails.Name}."; // <-- Note trailing '.' here

• MiniValidation/src/MiniValidation/MiniValidator.cs

  Line 247 in 7efcac2

  var itemPrefix = $"{prefix}[{index}].";

• MiniValidation/src/MiniValidation/MiniValidator.cs

  Line 288 in 7efcac2

  var key = $"{prefix}{memberName}";

• MiniValidation/src/MiniValidation/MiniValidator.cs

  Line 324 in 7efcac2

  errors.Add($"{prefix}{propertyName}", errorsList);