Rename core FixtureKit classes for simpler naming

[ngan]

Summary

• rename core classes to remove redundant Fixture prefixes:
  • FixtureKit::FixtureCache -> FixtureKit::Cache
  • FixtureKit::FixtureContext -> FixtureKit::DefinitionContext
  • FixtureKit::FixtureRegistry -> FixtureKit::Registry
  • FixtureKit::FixtureRunner -> FixtureKit::Runner
  • FixtureKit::FixtureSet -> FixtureKit::Repository
• rename corresponding files under lib/fixture_kit/
• update autoload paths in lib/fixture_kit.rb
• update call sites across library code, RSpec integration, helpers, specs, and README

Testing

• bundle exec rspec --fail-fast