Add to_smallvec for the slice primitive type

[sunjay]

The slice primitive type has a to_vec method which makes it convenient to convert a slice to a vector. It would be great if this crate could define an extension trait that added a method called to_smallvec (or some other name) which did the same thing for SmallVec.

This makes it particularly convenient to port code to using SmallVec since you can just import that extension trait instead of converting all the calls to to_vec. 😃

[gootorov]

As an alternative, it could be done using SmallVec::from_slice() method.

[sunjay]

Yes, that's what I'm doing now, but going through and replacing lots of to_vec calls is tedious. It would be nice if it was possible to just find and replace the method call. :)