Allow single-field named structs to be transparent

[Xiretza]

This more closely matches the criteria for e.g. #[repr(transparent)] and #[serde(transparent)].

TODO, if this is deemed a desirable change:

• Documentation updates
• Tests (Where should those go? The change isn't backend-specific)

Does your PR solve an issue?

Couldn't find an issue filed already.

Is this a breaking change?

No, code that previously compiled is unchanged, only code that used to be a compile error now does compile.

[abonander]

Looks fine with a couple of nits + docs and testing.

Tests (Where should those go? The change isn't backend-specific)

Preferably you should add a test for each backend to be sure the behavior is consistent. But if you don't have the time for that, I'd say just pick one:

• https://github.com/launchbadge/sqlx/blob/main/tests/mysql/derives.rs
• https://github.com/launchbadge/sqlx/blob/main/tests/postgres/derives.rs
• https://github.com/launchbadge/sqlx/blob/main/tests/sqlite/derives.rs

[abonander]

Thanks!