Add Derive for into_inner function

[vultix]

While there's no true IntoInner trait, it's a super common pattern for newtypes to provide an into_inner function. It'd be great if this crate could make it easy to add that function.

[vdods]

Seconded. The Into trait often requires specifying the type through some awkward syntax or temp value, so having an into_inner with a definite type would be great.

[ekanna]

Literally waiting for this. It will eliminate lot of code while passing sql params as tuple. We can write generic code for all tables.

[JelteF]

Sounds reasonable. I think it would fit well with the rest of our derives for static methods. PRs are welcome.