auto merge of #12039 : alexcrichton/rust/no-conditions, r=brson

This has been a long time coming. Conditions in rust were initially envisioned
as being a good alternative to error code return pattern. The idea is that all
errors are fatal-by-default, and you can opt-in to handling the error by
registering an error handler.

While sounding nice, conditions ended up having some unforseen shortcomings:

* Actually handling an error has some very awkward syntax:

        let mut result = None;
        let mut answer = None;
        io::io_error::cond.trap(|e| { result = Some(e) }).inside(|| {
            answer = Some(some_io_operation());
        });
        match result {
            Some(err) => { /* hit an I/O error */ }
            None => {
                let answer = answer.unwrap();
                /* deal with the result of I/O */
            }
        }

  This pattern can certainly use functions like io::result, but at its core
  actually handling conditions is fairly difficult

* The "zero value" of a function is often confusing. One of the main ideas
  behind using conditions was to change the signature of I/O functions. Instead
  of read_be_u32() returning a result, it returned a u32. Errors were notified
  via a condition, and if you caught the condition you understood that the "zero
  value" returned is actually a garbage value. These zero values are often
  difficult to understand, however.

  One case of this is the read_bytes() function. The function takes an integer
  length of the amount of bytes to read, and returns an array of that size. The
  array may actually be shorter, however, if an error occurred.

  Another case is fs::stat(). The theoretical "zero value" is a blank stat
  struct, but it's a little awkward to create and return a zero'd out stat
  struct on a call to stat().

  In general, the return value of functions that can raise error are much more
  natural when using a Result as opposed to an always-usable zero-value.

* Conditions impose a necessary runtime requirement on *all* I/O. In theory I/O
  is as simple as calling read() and write(), but using conditions imposed the
  restriction that a rust local task was required if you wanted to catch errors
  with I/O. While certainly an surmountable difficulty, this was always a bit of
  a thorn in the side of conditions.

* Functions raising conditions are not always clear that they are raising
  conditions. This suffers a similar problem to exceptions where you don't
  actually know whether a function raises a condition or not. The documentation
  likely explains, but if someone retroactively adds a condition to a function
  there's nothing forcing upstream users to acknowledge a new point of task
  failure.

* Libaries using I/O are not guaranteed to correctly raise on conditions when an
  error occurs. In developing various I/O libraries, it's much easier to just
  return `None` from a read rather than raising an error. The silent contract of
  "don't raise on EOF" was a little difficult to understand and threw a wrench
  into the answer of the question "when do I raise a condition?"

Many of these difficulties can be overcome through documentation, examples, and
general practice. In the end, all of these difficulties added together ended up
being too overwhelming and improving various aspects didn't end up helping that
much.

A result-based I/O error handling strategy also has shortcomings, but the
cognitive burden is much smaller. The tooling necessary to make this strategy as
usable as conditions were is much smaller than the tooling necessary for
conditions.

Perhaps conditions may manifest themselves as a future entity, but for now
we're going to remove them from the standard library.

Closes #9795
Closes #8968

diff --cc mk/tests.mk
Simple merge