Remove Chewy::Type, simplify DSL

[rabotyaga]

Remove the type concept from internals completely, as Elasticsearch is removing types:

• Remove the Chewy::Type class
  • e.g. remove CitiesIndex::City / CitiesIndex.city
    • CitiesIndex::City.import! ... becomes CitiesIndex.import! ...
• Simplify index DSL:
  • define_type block -> index_scope clause
    • it can be omitted completely, if you don't need to specify the scope or options, e.g. name
• Remove type names from string representations:
  • in update_index ActiveRecord helper and RSpec matcher, e.g.
    • update_index('cities#city') -> update_index('cities')
    • update_index(UsersIndex::User) -> update_index(UsersIndex)
  • in rake tasks (e.g. rake chewy:update[cities#city] -> rake chewy:update[cities])
  • in rake tasks output (e.g. Imported CitiesIndex::City in 1s, stats: index 3 -> Imported CitiesIndex in 1s, stats: index 3)
• Use index name instead of type name in loader additional scope
  • e.g. CitiesIndex.filter(...).load(city: {scope: City.where(...)}) -> CitiesIndex.filter(...).load(cities: {scope: City.where(...)})

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[dalthon]

Assuming that most of the work was based to move modules that once were included at Chewy::Type to Chewy::Index, and changing edge cases like index_scope, it LGTM!