Add #[must_use] for warnings

[durango]

@sgrif: I should add a #[must_use] on queries to prevent that mistake. Anyone mind opening an issue?

@sgrif: Making query.filter(name.like(user.name)); warn

[alexcameron89]

I think this is fixed and good to close.

[sgrif]

Yup. Thanks!

[TimNN]

Is this really fixed? The one PR linking this issue only mentions select and filter and also states:

But we should probably also add this to/think about insert, update and delete before we close that issue.

(I also just spend some time trying to figure out why my update wasn't executing until I noticed I was missing an .execute when adding an diesel::debug_sql. I'm pretty sure there were no warnings).

[sgrif]

Looks like we need an annotation on UpdateStatement and friends. Feel free to open a PR

[AndrewSpeed]

I'm happy to put a PR up for this, if no one else is working on it

[weiznich]

@AndrewSpeed Feel free to take it. If there is any problem just ask here or (preferable) in our gitter channel.