Remove Type<Postgres> from u32

[demurgos]

Sqlx currently implements Type<Postgres> on the u32 primitive (and related types):

sqlx/sqlx-core/src/postgres/types/int.rs

Line 77 in 9abe9b3

impl Type<Postgres> for u32 {

This is used as a shorthand when interacting with raw OIDs, but causes issues with regular code. Postgres does not support unsigned integers natively, but because of this impl it is easy to pass a u32 accidentally when binding values.

This is not a theorical footgun: it occurs to me every few months to get bitten by this.
Most recently, I had a bunch of code expecting a INT8 CHECK (VALUE >= 0) but forgot to use i64::from(some_input_u32) and managed to get a memory corruption on Postgres (because of unexpected argument buffer sizes).

The solution is simple: define an Oid(u32) wrapper type and implement the type-related traits on Oid instead of u32. It would make it more explicit when using a Oid and prevent these kinds of errors when using a bare u32.

EDIT: This is a breaking change, but could be part of the 6.0 0.6.0 release.

[jplatte]

Nitpick: The next release will be 0.6.0, not 6.0 😉

[paolobarbolini]

I've opened #1602 to try to get this fixed