path2: Adjust the API to remove all the _str mutation methods

Add a new trait BytesContainer that is implemented for both byte vectors
and strings.

Convert Path::from_vec and ::from_str to one function, Path::new().

Remove all the _str-suffixed mutation methods (push, join, with_*,
set_*) and modify the non-suffixed versions to use BytesContainer.

src/libstd/unstable/dynamic_lib.rs

@@ -121,7 +121,7 @@ mod test {
     fn test_errors_do_not_crash() {
         // Open /dev/null as a library to get an error, and make sure
         // that only causes an error, and not a crash.
-        let path = GenericPath::from_str("/dev/null");
+        let path = GenericPath::new("/dev/null");
         match DynamicLibrary::open(Some(&path)) {
             Err(_) => {}
             Ok(_) => fail2!("Successfully opened the empty library.")