Consider adding a `simple_query` method to `GenericClient`

[and-reas-se]

It seems like an oversight that the trait GenericClient doesn't have a simple_query method, when both types that implement it have that method. If you have a GenericClient you can call simple_query indirectly by starting a transaction and then calling it on the Transaction you get, but you can't just call it directly.

I ran into it when writing a function to do migrations. I wanted to start the transaction in the caller and have my function take a GenericClient,, but instead had to start the transaction inside the function to get access to the method in question.

Not a big deal, but I thought I'd point out the discrepancy.

[sfackler]

Huh, yeah it looks like it's missing from tokio-postgres's for some reason - strange! Happy to take a PR adding it, or I can do it when I have a chance.