std: Improve non-task-based usage

A few notable improvements were implemented to cut down on the number of aborts
triggered by the standard library when a local task is not found.

* Primarily, the unwinding functionality was restructured to support an unsafe
  top-level function, `try`. This function invokes a closure, capturing any
  failure which occurs inside of it. The purpose of this function is to be as
  lightweight of a "try block" as possible for rust, intended for use when the
  runtime is difficult to set up.

  This function is *not* meant to be used by normal rust code, nor should it be
  consider for use with normal rust code.

* When invoking spawn(), a `fail!()` is triggered rather than an abort.

* When invoking LocalIo::borrow(), which is transitively called by all I/O
  constructors, None is returned rather than aborting to indicate that there is
  no local I/O implementation.

* Invoking get() on a TLD key will return None if no task is available

* Invoking replace() on a TLD key will fail if no task is available.

A test case was also added showing the variety of things that you can do without
a runtime or task set up now. In general, this is just a refactoring to abort
less quickly in the standard library when a local task is not found.
auto merge of #14644 : alexcrichton/rust/more-no-runtime-use-cases, r=brson

A few notable improvements were implemented to cut down on the number of aborts
triggered by the standard library when a local task is not found.

* Primarily, the unwinding functionality was restructured to support an unsafe
  top-level function, `try`. This function invokes a closure, capturing any
  failure which occurs inside of it. The purpose of this function is to be as
  lightweight of a "try block" as possible for rust, intended for use when the
  runtime is difficult to set up.

  This function is *not* meant to be used by normal rust code, nor should it be
  consider for use with normal rust code.

* When invoking spawn(), a `fail!()` is triggered rather than an abort.

* When invoking LocalIo::borrow(), which is transitively called by all I/O
  constructors, None is returned rather than aborting to indicate that there is
  no local I/O implementation.

A test case was also added showing the variety of things that you can do without
a runtime or task set up now. In general, this is just a refactoring to abort
less quickly in the standard library when a local task is not found.