Refactor the code base

[ellnix]

Description
Rewrite much of the code base to make it easier to maintain and more stable.
This suggestion is only aimed at the code a user might interact with, I made a separate issue for refactoring the test suite: #300.

Basic example

• lib/meilisearch.rb is far too big to reason about
• Similarly named instance and class methods where one often just calls the other
• Return values are sort of random and inconsistent
• Methods often have side-effects
• Methods often have hacks waiting to break at any moment
• Logic is repeated in many places
• Some code is not reached by tests at all
• Difficult to add features such as Support for tenant tokens #152 Use index-swap when reindexing #267 or fix some issues like Skip indexCreation and settingsUpdate whenever adding new record in production #280
• Inconsistent public API when it comes to ! methods and method names in general
• Flags in method parameters (product.index!(true) does not express what true means)
• Poor error reporting (indexing can fail with no warning or return value provided to the user)
• There needs to be restructuring to integrate methods or classes to filter by an ActiveRecord (or other ORM) query, such as suggested in CanCan aware ms_search implementation #255
• There needs to be restructuring to integrate methods for searching multiple indexes with proper document-ORM conversion as required by Perform multiple searches in one single HTTP request #254 and Multiple documents for the same ActiveRecord object #283
• Add proper checks and validations to configuration (instead of the current implementation of the meilisearch method)

[ellnix]

Closed in favor of Milestone and #348