bindings-macro: get rid of the janky `is_integer_type` check wrt. sequences in favor of asserting a trait impl

[Centril]

Rather than this janky check that the type can be used as a sequence: https://github.com/clockworklabs/SpacetimeDB/blob/6e052cb8dc691adb0c894b5be68558676e246212/crates/bindings-macro/src/lib.rs#L565 let the sequence compatible types implement a trait and assert that it is implemented.