Relax memctx closures from Fn to FnOnce

[Hoverbear]

While working on #230 I was trying to use pgx::memcxt::PgMemoryContexts::exec_in_context and pgx::memcxt::PgMemoryContexts::switch_to and realized they used Fn over FnOnce.

This means that closures using it:

let internal = String::default();
PgMemoryContexts::TopTransactionContext.switch_to(|context| {
    let path = Path::from(internal); // <-- `internal` must be moved here by value, not ref.
});

Would get errors like: "cannot move out of internal, a captured variable in an Fn closure`"

This should not break anything for users, as it's a general relaxation.

• FnOnce: https://doc.rust-lang.org/std/ops/trait.FnOnce.html
• Fn: https://doc.rust-lang.org/std/ops/trait.Fn.html

Notably:

Since both Fn and FnMut are subtraits of FnOnce, any instance of Fn or FnMut can be used where a FnOnce is expected.

[eeeebbbbrrrr]

LGTM. Feel free to merge.