Use a temp table for encode_many

Just using IN will be faster for small numbers of crates, so you might
want to switch implementations depending on crates.len(). The best
solution would be to use COPY, but I haven't implemented that yet.

I haven't actually tested this, but it compiles!
Is this meant to build a query that looks like `... VALUES ($1$2$3...$n) ...` or should there be spaces/commas too?