Like `Cow<'_, B>`, but `B` is covariant
Implemented similar to [rust#96312]
Since `std::borrow::Cow` uses associated type, it invariant,
see [rust#115799] for details
[rust#96312]: https://github.com/rust-lang/rust/pull/96312
[rust#115799]: https://github.com/rust-lang/rust/issues/115799
# Examples
**Use `std::borrow::Cow` compile-fail case**:
```rust,compile_fail
use std::borrow::Cow;
fn foo<'a>(_: Cow<'a, [Cow<'a, str>]>) { }
fn bar<'a>(x: Cow<'a, [Cow<'static, str>]>) {
foo(x);
}
```
**Use `cow2::Cow` compile-passed case**:
```rust
use cow2::Cow;
fn foo<'a>(_: Cow<'a, [Cow<'a, str>]>) { }
fn bar<'a>(x: Cow<'a, [Cow<'static, str>]>) {
foo(x);
}
```