Request: conversion from Cow

It would be cool to have these methods
```rust
impl<'a, Buf> YarnBox<'a, Buf> {
    pub fn from_cow(c: Cow<'a, Buf>) -> Self {/***/}
}

impl<'a, Buf> From<Cow<'a, Buf>> for YarnBox<'a, Buf> {/***/}
```

Also, can I somehow find out the type of a `Yarn` value: borrowed, inlined, owned?
Thanks for the great and useful library!