[DeepSparse Evaluator API][UI] Simplify serialization

[dbogunowicz]

Feature Description

Introduces a few UI improvements to the Result serialization functionality. This includes:

• decoupling printing to CLI and saving the serialized results (printing to CLI will be covered in the separate PR)
• serializing the full Result object, not only Result.formatted
• turning save_evaluations into save_result and making sure it has only one responsibility (only saving, not returning data)
• updating and simultaneously simplifying the appropriate tests