[WIP] [Data API] Backend refactoring

[roaffix]

NOTE: the assumption that this PR will be merged right after the #261

Major changes:

• Wrap safe_call(backend.get()...) under the library call which is now used as a private wrapper of the C library. That makes the public part of the library more intuitive for both end-users and library contributors.
• Wrap ctypes under CType which inherits the ctypes._SimpleCData type.
• Project structure changes:
  • C library wrapper moved to the corresponding folder
  • Moved dtype aux functions to dtypes/helpers. Also, some methods were transferred there from the array_object file
• Added operators block which is duplicated the functionality of magic methods (like __add__) from the main array object and is required by the specification
• Changed testing structure to test parameterization and test auto-generation. These solutions decrease somehow an intuition of test cases but simplify the flow of adding new cases - it requires 1 or 2 rows instead of hundreds, now.

Minor changes:

• Removed some tags (TODO, FIXME, etc.) from the previous PR
• Removed an introduced in previous PR custom type ShapeType to avoid extra complexion in library typing
• Other minor code style changes
• Fixed minor bugs

--- WIP ---

• backend config separated into pre-setup and backend configuration settings
• add more operators
• add more tests coverage
• add mocks for different backend types for even more coverage of different test scenarios
• fix CI/CD in GitHub for CPU tests