Trigger support improvements

[Hoverbear]

Improves Trigger support roughly in line with #503 :

use pgx::{pg_trigger, pg_sys, PgHeapTuple, WhoAllocated, PgHeapTupleError, PgTrigger};

#[derive(thiserror::Error, Debug)]
enum CustomTriggerError {
    #[error("No current HeapTuple")]
    NoCurrentHeapTuple,
    #[error("pgx::PgHeapTupleError: {0}")]
    PgHeapTuple(PgHeapTupleError),
}

#[pg_trigger]
fn example_custom_error(trigger: &PgTrigger) -> Result<
    PgHeapTuple<'_, impl WhoAllocated<pg_sys::HeapTupleData>>,
    CustomTriggerError,
> {
    unsafe { trigger.current() }.ok_or(CustomTriggerError::NoCurrentHeapTuple)
}

[Hoverbear]

I'd like to add some more tests...

[eeeebbbbrrrr]

fn trigger_example<'a>(trigger: &'a pgx::PgTrigger<'a>) -> Result<Option<PgHeapTuple<'a, impl WhoAllocated<pgx::pg_sys::HeapTupleData>>>, TriggerError> {

That's special. Wonder if we could make a type alias like:

type TriggerResult = PgHeapTuple<'a, impl WhoAllocated<pgx::pg_sys::HeapTupleData>

[Hoverbear]

You can't use impl Trait in type aliases sadly.

[Hoverbear]

(I removed the extra lifetimes from the siganture when I was testing now)

[Hoverbear]

What's your take on the safety of this one, @eeeebbbbrrrr ?