Add RSpec's subject interface including: - [x] [explicit #subject](https://www.relishapp.com/rspec/rspec-core/v/3-3/docs/subject/explicit-subject) - [x] [implicit #subject](https://www.relishapp.com/rspec/rspec-core/v/3-3/docs/subject/implicitly-defined-subject) - [x] [#described_class](https://www.relishapp.com/rspec/rspec-core/v/3-3/docs/metadata/described-class) - [ ] [#let](https://www.relishapp.com/rspec/rspec-core/v/3-3/docs/helper-methods/let-and-let) - [ ] [One-liner syntax](https://www.relishapp.com/rspec/rspec-core/v/3-3/docs/subject/one-liner-syntax)
