Implement Eq, Ord, PartialOrd, Hash for CxxString

[jovenlin0527]

Closes #737.

This is implemented by delegation to &[u8], as suggested.

I also implemented PartialOrd<str> for CxxString and PartialOrd<CxxString> for str, since PartialEq<str> is already implemented for CxxString and vice versa. I added tests for comparisons between str and CxxString, just in case.

[jovenlin0527]

Oh by the way the standard library implements Hash for str differently. https://doc.rust-lang.org/src/core/hash/mod.rs.html#598-603. They add additional 0xff when hashing.
Do we follow the standard library? If we don't, then we should reconsider implementing PartialEq<str> for CxxStrins and some other traits.

[dtolnay]

Thanks!

[dtolnay]

If we don't, then we should reconsider implementing PartialEq<str> for CxxString and some other traits.

Why? I don't know of any relationship that PartialEq would impose on the hash implementation.

[jovenlin0527]

If we don't, then we should reconsider implementing PartialEq<str> for CxxString and some other traits.

Why? I don't know of any relationship that PartialEq would impose on the hash implementation.

I thought if two objects are equal, then they should have same hash? I may be mistaken though.

[dtolnay]

That is definitely not required.