Add `transfer_buffer_length` to `handle_urb`

[h7x4]

Closes #24.

This breaks the API. Since #18 is also breaking, could I ask both to be released under a new major version when they eventually get reviewed and merged?

Also, should we enforce people to conform to the given buffer length by providing a mutable slice and making handle_urb -> Result<()>, rather than the comment?

[jiegec]

Thanks, for breaking changes, we can write a changelog to help migration.