uniq: non-UTF-8 file names and fewer copies

[jeddenlea]

uniq ought to be able to work with files whose names are not UTF-8.

I've also taken the chance to remove some unnecessary String allocations.

Additionally, both open_input_file and open_output_file were returning items with two implied levels of Box<dyn ...> that were not necessary.

In the case of stdin as input, it was not necessary to wrap stdin with a BufReader, StdinLock implements BufRead itself.

[tertsdiepraam]

Looks good! I think we should probably make some helper for this in uucore. Though that should maybe be implemented with an enum instead like in dd:

coreutils/src/uu/dd/src/dd.rs

Line 109 in 4cc569f

enum Source {

Could be an interesting thing to look into next if you want :)