Change `Bindings::write()` to take `impl Write` instead of `&mut dyn Write`

[MarijnS95]

Noticed immediately after #3317 was merged:

Dynamic dispatch is not (always) needed here, and this way an owned File can trivially be moved into the function again like before without forcing a mutable borrow. Noting that Write is even implemented on &File, which can now be more easily passed without having it under mut.