API changes

• library switched to non-consuming API:

  // pre 0.24.0:
  let conn = conn.query_drop("DO 1").await?;
  
  // 0.24.0:
  conn.query_drop("DO 1").await?;
  

• Queryable trait was updated and now it is more convenient to work with usual single-result sets. Use Queryable::query_iter and Queryable::exec_iter if you need more flexibility.

• Query trait was introduced. It allows you to run queries directly on a Pool and to create 'static QueryResults

• Thick Stmt structure was removed in favor of the Statement structure, which is a thin wrapper over raw statement identifier and metadata.

Fixes

• #110