Add From impls for CowArray, similar to Array and ArrayView

[jturner314]

Discussed in https://github.com/rust-ndarray/ndarray/discussions/1036

^{Originally posted by hombit June 18, 2021} Is there any reason why CowArray doesn't implement all From which is implemented by both Array and ArrayView? My current usage is limited to build CowArray from both &Vec and Vec, but generally I'd like to write something like:

struct S<'a> {
    ca: CowArray<'a, f64, Ix1>
}

impl<'a> S<'a> {
    fn new(a: impl Into<CowArray<'a, f64, Ix1>>) -> Self {
        Self {ca: a.into()}
    }
}

[jturner314]

Two PRs will resolve this:

• Conversions from owned types: #1028
• Conversions from slices/references: #1038