Remove mach_sty
libcore: Export core::from_str::FromStr from core::prelude
Remove codes related to modes
libstd: De-mut arena
librustc: Update the serializer to work properly with INHTWAMA, removing mutable fields in the process
librustc: Make uninhabited enums not castable to int
auto merge of #6201 : pcwalton/rust/inhtwama-serializer, r=graydon

This PR removes mutable fields from the serializer and makes the encoder and decoder use INHTWAMA properly (i.e. `&mut self`).

r? @graydon
auto merge of #6206 : sanxiyn/rust/mach-sty, r=thestinger
std: xfail test_serializing_pipes
auto merge of #6212 : brson/rust/xfail-flatpipes, r=brson

This is preventing a snapshot. Filed #6211
Remove extra `#[cfg(stage0)]`
auto merge of #6204 : pcwalton/rust/uninhabited-enum-cast, r=catamorphism

r? @catamorphism
auto merge of #6207 : sanxiyn/rust/tc-big, r=thestinger
auto merge of #6185 : gifnksm/rust/prelude-from_str, r=graydon

`core::prelude`  re-exports `core::to_str::ToStr`, but doesn't re-export `core::from_str::FromStr`.
That is inconsistent.
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`.
core::rt: Remove Close trait

We will just use RAII for now.
core::rt: Fix a warning about unnecessary mutable variable
core::rt: Begin implementing TcpStream

This ended up touching a lot of code related to error handling.
core::rt: Clean up the interface to rtio

Make names that better match rt::io. Return error types.
core::rt: Improve docs