Refactor `meilisearch.rb`

[ellnix]

• Streamline interface
  • remove similarly named instance and class methods where one often just calls the other
  • reduce method side effects
  • reduce hacks in methods
  • clarify bang method API
  • do not use flag parameters (i. e. product.index!(true))
  • make return types more consistent
• Reach 95%+ code coverage
• More robust error reporting (no failing silently)
• Break meilisearch.rb into multiple files/modules
• Add a system for validating configurations