Add a container-level `rename_all` attribute to `FromSql`/`ToSql`

[jakubadamw]

It works both with composite types' fields and enum variants, although arguably it will be the most useful for the latter as that's where the naming conventions between Rust and a typical PostgreSQL schema differ.

Right now it supports the following casing styles, following serde:

• "camelCase"
• "kebab-case"
• "lowercase"
• "PascalCase"
• "SCREAMING-KEBAB-CASE"
• "SCREAMING_SNAKE_CASE"
• "snake_case"
• "UPPERCASE"

Addresses #934.

[jakubadamw]

So this adds a few dependencies to keep the implementation concise. I, personally, subscribe to the “use as much of the crates.io ecosystem as you can possibly can” philosophy, but I can see how it may not be appropriate for the project. Still, that's what I did in the first version to gather feedback. Which dependency should be got rid of can perhaps be decided later.

[allan2]

Since this is a library, I think it would be better to avoid introducing external dependencies to keep compile times down.

@sfackler thoughts?

The case conversion code in serde_derive might be helpful.

[JaydenElliott]

@jakubadamw are you still working on this, would love this feature

[jakubadamw]

@JaydenElliott hey, not really, no. I'll be happy for someone else to take over this. 🙂