auto merge of #6046 : brson/rust/io, r=graydon

r? @pcwalton

Sorry this is so big, and sorry the first commit is just titled 'wip'.

Some interesting bits

* [LocalServices](https://github.com/brson/rust/commit/f9069baa70ea78117f2087fe6e359fb2ea0ae16a) - This is the set of runtime capabilities that *all* Rust code should expect access to, including the local heap, GC, logging, unwinding.
* [impl Reader, etc. for Option](https://github.com/brson/rust/commit/5fbb0949a53a6ac51c6d9b187ef4c464e52ae536) - Constructors like `File::open` return Option<FileStream>. This lets you write I/O code without ever unwrapping an option.

This series adds a lot of [documentation](https://github.com/brson/rust/blob/io/src/libcore/rt/io/mod.rs#L11) to `core::rt::io`.

diff --cc src/libcore/core.rc
Simple merge