v5.2.0

• first and first_exec shortcuts added to conveniently get first row of a query

• The lifetime of a pool is now untied from a QueryResult in Pool::prepare and Pool::prep_exec so now you can write this without bc errors:

  pool.prep_exec("SELECT 1", ()).and_then(|result1| {
      pool.prep_exec("SELECT 2", ()).and_then(|result2| {
          // ...
      })
  })