fix: Escape enum members containing reserved keywords

[jacobsvante]

Just stumbled upon an issue where one of the enum members of an existing postgres enum was box. Cornucopia happily generated the cornucopia.rs file, but the result was a file with a syntax error in it.

So here comes my quick-fix to that.

This PR is not meant to be merged, just demonstrating one way to resolve the issue. I'm a bit short on time so if anyone with more intimate knowledge of the codebase wants to do it properly, then please go ahead 😉

[jacobsvante]

#[allow(non_camel_case_types)] needs to be added to pub mod types, to avoid warnings.

[jacobsvante]

Hmmm. It seems that postgres/tokio-postgres does not like enums with raw identifiers. Gives me the following error when I try to execute a SELECT with the given column included:

thread 'main' panicked at 'error retrieving column 9: error deserializing column 9: cannot convert between
the Rust type `data_import::cornucopia::types::public::Uom` and the Postgres type `uom`',
/Users/jacob/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-postgres-0.7.7/src/row.rs:151:25

The enum looks like this:

        pub enum Uom {
            pair,
            pcs,
            r#box,
        }

If I do ALTER TYPE uom RENAME VALUE 'box' TO 'Box' and re-run cornucopia, the issue disappears.

[jacobsvante]

Replaced by more thorough version in #161