minor: const array bson conversion

[abr-egn]

Where functions have signatures like fn foo(value: impl Into<Bson>) this enables the nice foo([1, 2, 3]) where currently it needs the awkward foo([1, 2, 3].as_ref()) or similar workaround.

For things accepting arrays, usually you can just use foo(&[1, 2, 3]) but that doesn't work here because the argument is actually of type &[i32; 3]; the compiler will coerce the ref to &[i32] only in the context of a concrete type, not when matching against trait impls :(