Since 0.4.1
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- Added Executor trait
  -> Moved query, insert_..., update, delete out of internal
     They can now be expressed without sqlx, using only QueryStrategy
     and Executor (which is implemented for &db and &mut transaction)
- Exposed querys' futures' types to allow wrapping without boxing
  -> Removed unnecessary internal boxing of futures/streams
     by not using some of sqlx::Executor's convenience methods
