Diesel: Add support for `HAVING` clauses

[weiznich]

With #2571 merged diesel now officially supports GROUP BY causes. As mentioned in #210 we have no concrete planes for supporting HAVING clauses in the next release. I've opening this issue to just have one for HAVING clauses + write down some notes for potential contributors. So if anyone is interested in adding support for such clauses feel free to open a PR.

Steps to add support for HAVING clauses:

• Define a HavingDsl trait similar to the existing FilterDsl trait
• Add a generic impl for all tables, similar to this impl for FilterDsl.
• Add support for HAVING clauses to our query builder:
  
  
  
  • Add a NoHavingClause similar to NoWhereClause
  
  
  • Add a HavingClause<Expr> similar to WhereClause<Expr>
  
  
  • Add another generic field to SelectStatement containing the having clause. Default that generic parameter to NoHavingClause. Add this generic parameter to all impls in that file.
  
  
  • Make all of the impls in `diesel/src/query_builder/select_statement/dsl_impl.rs generic over the new generic paramater.
  
  
  • Fix all compilation errors coming from the newly added field by forwarding the having clause to the newly constructed select statement structs.
  
  
  • Implement HavingDsl for SelectStatement
  
  
  • Add another boxed field to BoxedSelectStatement containing the boxed HAVING clause
  
  
  • Implement HavingDsl for SelectStatement
  
  
• Add a helper type to diesel::dsl (Similar to diesel::dsl::Filter<Source, Filter>
• Add a having method + some documentation to QueryDsl, similar to filter()
• Add a few tests to diesel_tests

[dnamsons]

I would like to work on this issue

[weiznich]

@dnamsons Sure, go for it. If you hit any issue feel free to reach out for me here or in our gitter channel. Opening a WIP PR is also probably a good idea to get early feedback on the implementation.