std: implement fileinput.

Iterate over lines in a series of files. API (mostly) adopted from
Python's fileinput module.
auto merge of #5202 : dbaupp/rust/std-fileinput, r=graydon

As per https://github.com/mozilla/rust/wiki/Note-wanted-libraries.

Iterates over lines in a series of files, e.g. a basic `cat`

```rust
use std::fileinput;
fn main() {
    for fileinput::input |line| {
        io::println(line);
    }   
}
```

The API is essentially a subset of [Python's fileinput module](http://docs.python.org/3.3/library/fileinput.html), although the lack of default arguments and global mutable state means that there are extra functions to handle a few different cases (files from command line arguments, files from a vector, accessing current filename/line number).

A few points that possibly require adjustment:
- Most functions take vectors of `Path` (well, `Option<Path>`) rather than just `~str`, since this seems safer, and allows finer control without the number of different functions/methods increasing exponentially.
- `pathify` has a stupid name.
- I'm not quite sure how to mock tests that require external files: the tests in `libcore/io.rs` seem to indicate using a `tmp` subdirectory, so that's what I did, but I can't reliably build rust on this computer to test (sorry! although I have run the tests in just `fileinput.rs`  after creating `./tmp/` manually).
- The documentation I've written seems pretty crappy and not particularly clear.
- Only UTF8 files are supported.