QueryHelper::filter should take a ConditionWhere as argument

[nitnelave]

Right now, filter takes a simpleExpr as argument, which can be combined into more complex boolean statements with .and and .or, but I think it would be a better interface (and clearer, and more consistent) if it took a sea_query::query::condition::ConditionWhere (or rather an Into<ConditionWhere>).

[tyt2y3]

Thank you for your suggestion. But may be it is not the best to 'overload' filter, because there is a semantic difference: filter() can be called many times, but condition() can be called once only.
I tried to use 'compile time state machine' to guard against this, but it inevitably complicates the interface.
So may be I keep it simple for now.

[tyt2y3]

I think I solved this problem. By allowing cond_where to be called multiple times (SeaQL/sea-query@ef6ac3b), and_where and cond_where actually behaves the same now.