api: add impl<'a> From<&'a BString> for Cow<'a, BStr>

This commit simplifies code in situations like:

```
let mut v = Vec::<Cow<'a, BStr>>::new();
let s = BString::new(...);

// Before this commit, we would have to do:
// v.push(s.as_bstr().into());
v.push(s.into());
```

PR #187

src/impls.rs

@@ -268,6 +268,13 @@ mod bstring {
  }
  }
 
+ impl<'a> From<&'a BString> for Cow<'a, BStr> {
+ #[inline]
+ fn from(s: &'a BString) -> Cow<'a, BStr> {
+ Cow::Borrowed(s.as_bstr())
+ }
+ }
+
  impl TryFrom<BString> for String {
  type Error = crate::FromUtf8Error;