Remove Chewy::Type, simplify DSL: (toptal#783)

- 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(...)})`

spec/chewy/fields/base_spec.rb

@@ -145,7 +145,6 @@
       end
 
       specify do
-<<<<<<< HEAD
         expect(EventsIndex.mappings_hash).to eq(
           mappings: {
             properties: {
@@ -160,20 +159,6 @@
                       id: {type: 'integer'},
                       created_at: {type: 'time'}
                     }
-=======
-        expect(EventsIndex::Event.mappings_hash).to eq(
-          properties: {
-            id: {type: 'integer'},
-            category: {
-              type: 'object',
-              properties: {
-                id: {type: 'integer'},
-                licenses: {
-                  type: 'object',
-                  properties: {
-                    id: {type: 'integer'},
-                    created_at: {type: 'time'}
->>>>>>> e3d155a (Drop support for Elasticsearch 6.x, prepare 7.1.0 release (#766))
                   }
                 }
               }
@@ -345,7 +330,6 @@
       end
 
       specify do
-<<<<<<< HEAD
         expect(EventsIndex.mappings_hash).to eq(
           mappings: {
             properties: {
@@ -358,18 +342,6 @@
               },
               category: {type: 'object'}
             }
-=======
-        expect(EventsIndex::Event.mappings_hash).to eq(
-          properties: {
-            id: {type: 'integer'},
-            name: {
-              type: 'integer',
-              fields: {
-                raw: {analyzer: 'my_own', type: Chewy.default_field_type}
-              }
-            },
-            category: {type: 'object'}
->>>>>>> e3d155a (Drop support for Elasticsearch 6.x, prepare 7.1.0 release (#766))
           }
         )
       end