Move the `finalize()` function out of the `impl Output` block and up
to the module level so that it can be accessed by other module-level
functions.

Move the `dd_out()` function out of the `impl Output` and up to the
module level, renaming it to `dd_copy()`. This change aligns it with
the pattern set by `std::io::copy()`, which similarly takes an input
reader and an output writer.

This does not change the behavior of `dd`, just the code organization
to make it more closely match the idioms in the Rust standard library.