Rename last to last_opt, last_unsafe to last

As per discussion on IRC. I am about to file an RFC for further
discussion about the more general issue of whether to enforce
invariants through types, typestate, or dynamic checks, but for now,
removing the misleading name "last_unsafe".

src/libstd/fs.rs

@@ -271,7 +271,7 @@ fn splitext(p: path) -> (str, str) {
         if vec::len(parts) > 1u {
             let base = str::connect(vec::init(parts), ".");
             // We just checked that parts is non-empty
-            let ext = "." + vec::last_unsafe(parts);
+            let ext = "." + vec::last(parts);
 
             fn is_dotfile(base: str) -> bool {
                 str::is_empty(base)