Also bump Rust requirement to 1.25 which is a requirement of that feature

Derive Clone, Copy, Eq, Hash, and PartialEq for all types. Not all
traits are supported by all types, which is why many are missing
some.

As this is now derived, having a test is unnecessary

Abstract paths should always be N-1 in length where N is the length of
the `sun_path` field (first byte is \0). Given that,
`UnixAddr::new_abstract()` should always return this N-1 length, not
just the length of the string provided (the rest of the array will be
\0s).

This is a minor optimization that allows for reduced sizes of datatypes. Since this pointer
will never be NULL, it's safe to use here