mark some functions as returning !

Closes #10023
auto merge of #10042 : thestinger/rust/fail, r=sanxiyn

Closes #10023
Cleaned, documented, wrote tests for up std::bool
Removed unused import warning in std::mem and cleaned it up too

Removed is_true and is_false from std::bool

Removed freestanding functions in std::bool
auto merge of #10007 : Kimundi/rust/add_docs_01, r=cmr

Additionally:
- Sorted the prelude reexports
- Removed unused import warning in std::mem and cleaned it up too

(I was bored :ghost: )
libextra: Make arc clone inline
libextra: Make arc::get and arc::new inline too.

Should be a 2x improvement in a Servo test case.
Implement io::net::unix
Finish implementing io::net::addrinfo

This fills in the `hints` structure and exposes libuv's full functionality for
doing dns lookups.
Address a few XXX comments throughout the runtime

* Implement Seek for Option<Seek>
* Remove outdated comment for io::process
* De-pub a component which didn't need to be pub
Remove unbound pipes from io::pipe

This isn't necessary for creating processes (or at least not right now), and
it inherently attempts to expose implementation details.
Move rt::io::stdio from FileStream to a TTY

We get a little more functionality from libuv for these kinds of streams (things
like terminal dimentions), and it also appears to more gracefully handle the
stream being a window. Beforehand, if you used stdio and hit CTRL+d on a
process, libuv would continually return 0-length successful reads instead of
interpreting that the stream was closed.

I was hoping to be able to write tests for this, but currently the testing
infrastructure doesn't allow tests with a stdin and a stdout, but this has been
manually tested! (not that it means much)
Migrate Rtio objects to true trait objects

This moves as many as I could over to ~Trait instead of ~Typedef. The only
remaining one is the IoFactoryObject which should be coming soon...
Remove rt::io::support

This removes the PathLike trait associated with this "support module". This is
yet another "container of bytes" trait, so I didn't want to duplicate what
already exists throughout libstd. In actuality, we're going to pass of C strings
to the libuv APIs, so instead the arguments are now bound with the 'ToCStr'
trait instead.

Additionally, a layer of complexity was removed by immediately converting these
type-generic parameters into CStrings to get handed off to libuv apis.
Remove IoFactoryObject for ~IoFactory

This involved changing a fair amount of code, rooted in how we access the local
IoFactory instance. I added a helper method to the rtio module to access the
optional local IoFactory. This is different than before in which it was assumed
that a local IoFactory was *always* present. Now, a separate io_error is raised
when an IoFactory is not present, yet I/O is requested.
Don't attempt to export uv functions directly
Migrate the last typedefs to ~Trait in rtio

There are no longer any remnants of typedefs, and everything is now built on
true trait objects.
Move as much I/O as possible off of native::io

When uv's TTY I/O is used for the stdio streams, the file descriptors are put
into a non-blocking mode. This means that other concurrent writes to the same
stream can fail with EAGAIN or EWOULDBLOCK. By all I/O to event-loop I/O, we
avoid this error.

There is one location which cannot move, which is the runtime's dumb_println
function. This was implemented to handle the EAGAIN and EWOULDBLOCK errors and
simply retry again and again.
Remove even more of std::io

Big fish fried here:

    extra::json
    most of the compiler
    extra::io_util removed
    extra::fileinput removed

Fish left to fry

    extra::ebml
Stop logging task failure to task loggers

The isn't an ideal patch, and the comment why is in the code. Basically uvio
uses task::unkillable which touches the kill flag for a task, and if the task is
failing due to mismangement of the kill flag, then there will be serious
problems when the task tries to print that it's failing.
Remove io::read_error

The general idea is to remove conditions completely from I/O, so in the meantime
remove the read_error condition to mean the same thing as the io_error condition.