Consider replacing `ToString` bound for more precise bound

[pcpthm]

Currently, query builder accepts ToString for its arguments. But this trait doesn't have definite semantics and many types implement this. Most of the instances are irrelevant to database query, such as error types (because it implements Display for human display of the error).
I'm not sure why just accepting String or impl Into<String> is not enough. Probably it should be.

[Drevoed]

I share your thoughts about Into<String>, I have already converted usages of ToString to it in my wip branch.

[Empty2k12]

The idea was accepting both &str and String. I agree that using Into<String> is better.