Replace unmaintained encoding dependency with encoding_rs

[rhysd]

bat depends on encoding crate but it is no longer maintained. This is reported as RUSTSEC-2021-0153 in the advisory DB. The advisory suggests encoding_rs crate as alternative.

This PR replaces the unmaintained crate with the alternative.

As a side effect, this patch makes bat's UTF16 support a bit more efficient:

• Decoding with UTF16LE or UTF16BE is now handled with Cow instead of String. It means that it can avoid extra allocation by reusing the original string buffer when no character in the buffer was replaced while encoding.
• Removing BOM is now handled by encoding_rs more efficiently thanks to encode_with_bom_removal methods. The previous implementation always allocated new buffer only for removing BOM.

[sharkdp]

Thank you very much for your contribution!

Also great to see that this makes the code easier.