Implement `PartialEq` between `Owned` and `View`

[japaric]

Blocked on: rust-lang/rfcs#439. This needs PartialEq that accepts an arguments of different types.

All these should work:

• Mat == View/MutView
• Col<Box[T]>> == Col<[strided]Slice>
• Row<Box[T]>> == Row<[strided]Slice>
• and vice versa