Consider refactoring `*const T` to `NonNull<T>` or `&T`

[workingjubilee]

These files use *const to some T when they could probably actually use either:

• NonNull<T>
• &T
• Option<NonNull<T>>
• Option<&T>

The Optional forms of these types should be bit-equivalent to *const T. If in doubt, skip any cases where this conversion might seem incorrect due to a wonky lifetime concern, or use a more conservative form.

• [ ] pgx/src/htup.rs
• [ ] pgx/src/varlena.rs
• [ ] pgx/src/itemptr.rs
• [ ] pgx/src/guc.rs
• [ ] pgx/src/datum/uuid.rs
• [ ] pgx/src/datum/varlena.rs
• [ ] pgx/src/datum/internal.rs
• [x] pgx/src/datum/array.rs
• [ ] pgx/src/datum/json.rs
• [ ] pgx/src/hooks.rs
• [ ] pgx/src/stringinfo.rs
• [ ] pgx/src/bgworkers.rs
• [ ] pgx/src/enum_helper.rs
• [ ] pgx/src/memcxt.rs
• [ ] pgx/src/log.rs

[therewillbecode]

Thinking about picking this up. Is it still needed?