Mark query builder functions as const function

[Evian-Zhang]

Motivation

I'm new to this crate. and during my experience of writing ORMs, I think it is wise to mark query builder functions as const functions. In most situations, one query corresponds to one static string, which we can define them in const environments. However, current implementation of query builders does not contain a const mark, while I didn't find something prevents doing so.

Proposed Solutions

Mark query builder functions as const function.