Allow Cow<'a, [u8]> to be deserialized even when visitor receives owned bytes.

[zakarumych]

Currently Cow<'a, [u8]> uses &'a [u8] deserialization implementation which would fail when deserializing from any non-binary format.
While this is expected for &'a [u8] but Cow<'a, [u8]> can contain Vec<u8> as well and doesn't have to be a pointer into deserializer owned data.

I suggest to write a special visitor for Cow<'a, [u8]> that will create Cow::Borrowed when visit_borrowed_bytes or visit_borrowed_str is called, and Cow::Owned for visit_str, visit_string, visit_bytes or visit_byte_buf.

[zakarumych]

Probably the same must be done for Cow<'a, Bytes>

[dtolnay]

I would accept a PR to fix this.

[timotree3]

I have code for this and am working on a PR now.