make Array<&str> serializable via per-type Serialize impls

[isdaniel]

Motivation

The blanket impl<T: UnboxDatum> Serialize for Array<'mcx, T> is unreachable for reference element types: the UnboxDatum::As<'src> where Self: 'src combined with the for<'arr> HRTB forces 'mcx: 'static, so Array<&str>: Serialize never resolves. This is why array_tests.rs carries the TODO disabling serde_serialize_array, and why extensions returning JSON over text[] are forced into Vec and lose zero-copy.

How to use

#[pg_extern]
fn serde_serialize_array<'dat>(values: Array<'dat, &'dat str>) -> Json {
    Json(json! { { "values": values } })
}

Breaking change

Downstream UnboxDatum types (custom type) that relied on the blanket to auto-derive Serialize for Array now need a one-line impl:

impl<'mcx> Serialize for Array<'mcx, MyType> {
  fn serialize<S: Serializer>(&self, s: S) -> Result<S::Ok, S::Error> {
	  s.collect_seq(self.iter())
  }
}

Stable Rust has no specialization, so adding Array<&str> impls required dropping the blanket. All in-tree primitive types stay
covered.