Bump rustc-serialize and use std::path

[alexcrichton]

This commit bumps the dependency on rustc-serialize to 0.3 and allows
Url::from_file_path to work with std::path::Path as well (transitively
working for PathBuf too).

[alexcrichton]

Er sorry, forgot to fill out some other portions, adding tests and such now.

[alexcrichton]

Ok! Filled out tests and verified that it's working on both unix and windows

[SimonSapin]

Is it still useful to have both Url::form_file_path and Url::form_directory_path, in the new std::path, or does Path now record enough information to tell them apart? (This can be a follow up.)

It's unfortunate that we now can't manipulate/test paths for platforms other than the one we build for (compared to using e.g. std::old_path::windows:: Path directly). But I guess the complexity of multiple types and traits is not worth it.

[alexcrichton]

Currently yes I believe that the distinction is available. Very little normalization happens by default so a is a distinct path from a/ (which I think is what you want, right?).

For example: http://is.gd/l1SS8I

[SimonSapin]

Right, this is what I meant. But I don’t know if we can rely on things like std::env::current_dir (when it’s ported to new path), or other user code that constructs a path to a directory, to include that CurDir component.

[alexcrichton]

Ah yes that I would not rely on (I doubt current_dir returns paths with trailing slashes)

[SimonSapin]

Ok, so we’ll keep the separate from_*_path methods.

Thanks for the PR!