libcollection: generalize StrVector to AsSlice<Str>.

[nodakai]

A single impl supports all of [T], Vec<T> and CVec<T>.

Once Iterable is implemented, we will prefer it to SlicePrelude.
But the with_capacity() part might become tricky.

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[alexcrichton]

In general the *Prelude traits are not intended to be generalized over, they're purely intended to provide the ability to define inherent methods on language-defined types. Could you elaborate a little more on the motivation for this patch? You mention that [T], Vec<T> and CVec<T> all benefit, but [T] and Vec<T> should both benefit today to to the existing impl and the Deref impl on Vec. I suspect that the CVec<T> type needs to itself be revisited and redesigned, so it may not be the best type to optimize for right now.

[nodakai]

@alexcrichton I eventually found a minimum change to generalize the existing impl for Vec<Str> to AsSlice<Str> was the best. User-defined types implementing AsSlice<Str> will also benefit from StrVector methods.

Granted, it's only one method call away (mycontainer.as_slice().concat().) But since this change won't break anything and is an incremental refine, I think it's worth getting into the stdlib.