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:

SpacetimeDB/crates/bindings-macro/src/lib.rs

Line 565 in 6e052cb

&& !matches!(field.ty, syn::Type::Path(p) if is_integer_type(&p.path))

let the sequence compatible types implement a trait and assert that it is implemented.

[Centril]

This doesn't seem to apply anymore / it is done.