Solution: implement an interface for sub-transactions

It's a fairly simplistic interface at this point and it allows choosing
a mode of release on drop (commit by default).

This is an improved version of the original implementation in
pgx-contrib-spiext
(https://github.com/supabase/pgx-contrib-spiext/blob/main/src/subtxn.rs)

It may be unnecessary.

Solution: simply derive it

Solution: name it Parent as it is what it is

Also, given `PgMemoryContexts` API is still somewhat rough, it may be
not a great idea to expose it.

Solution: remove it

…is for

Solution: comment it

Solution: provide some reference materials on it

It is only there so that we can move it out of manually copied type
that implements Drop

Solution: extract release logic into a separate trait

Solution: document them

We can do better

Solution: export only most common types