rename vec::from_slice and/or str::to_unique

[jesse99]

These do exactly the same thing so it would be nice if they had similar names (I guess this would require making the vector version a method).

[catamorphism]

to_unique is to_owned now, but from_slice is still the same.

[brson]

Not 0.6

[nickdesaulniers]

What's the status of this bug?

[huonw]

Fixed by #6389: from_slice was renamed to to_owned.