text: Title: Add `__slots__` definitions
  Body: ## Problem
  
  <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  We currently don't define the `__slots__` attribute for any of our classes. Adding it is a free performance improvement.
  
  ## Suggested solution
  
  <!-- A clear and concise description of what you want to happen. -->
  
  We should add `__slots__` definitions to as many classes as possible. https://github.com/ariebovenberg/slotscheck/ should then be included in tests to ensure we are defining the slots correctly.
  
  Non-exhaustive list:
  - `client.Prisma`
  - `actions.*Actions`
  - `http_abstract.HTTP`
  - `http_abstract.Response`
  
  I am unsure if we can/should add slots to model classes.
  