StatementLike lifetime consistency in Queryable

[ch3pjw]

It appears that the lifetime of Q: StatementLike + 'a in Queryable::exec_iter is at odds with that in, for example Queryable::exec where Q: StatementLike + 'b. In the first case the 'a is the lifetime of the Queryable thing (the connection, transaction or so forth), whilst in the second case the 'b is the lifetime of the StatementLike thing itself.

It feels to me like the correct lifetime for the StatementLike should be it's own lifetime. This PR proposes that. This does have a material effect on some code I'm writing elsewhere - with the overly constrained lifetimes before this PR, I effectively have to make 'a and 'b the same in my code.

An additional question would be that exec_iter uses 's as the lifetime for the StatementLike, which I find a lot more readable than the 'b used elsewhere. Could/should it be updated to be 's everywhere for consistency?

Note the PR also suppresses a Clippy waning, just so that I was starting from a lean slate before my changes.