collections: optimize `HashMap`. Add `DefaultResizePolicy`.

Refactored the load factor and the minimum capacity out of HashMap.
The size of HashMap<K, V> is now 64 bytes by default on a 64-bit platform
(or 48 bytes, that is 2 words less, with FNV)
Added a documentation in a few places to clarify the behavior.
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.
Fixes #14185.

This fix suppresses dead_code warnings from code generated by regex! when
the result of regex! is unused. Correct behavior should be a single
unused variable warning.

Regression tests are included for both `let` and `static` bound regex!
values.
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.
auto merge of #14526 : pczarn/rust/hashmap-opt, r=alexcrichton

An interface that gives a better control over the load factor and the minimum capacity for HashMap.
The size of `HashMap<K, V>` is now 64 bytes by default on a 64-bit platform (or at least 40 bytes, that is 3 words less, with FNV and without minimum capacity)

Unanswered questions about `ResizePolicy`

* should it control the `INITIAL_CAPACITY`?
* should it fully control the resizing behavior? Even though the capacity always changes by a factor of 2.
* is caching `grow_at` desirable?

special thanks to @eddyb and @pnkfelix
auto merge of #14647 : BurntSushi/rust/fix-14185, r=alexcrichton

This fix suppresses dead_code warnings from code generated by regex! when
the result of regex! is unused. Correct behavior should be a single
unused variable warning.

Regression tests are included for both `let` and `static` bound regex!
values.

see #14185
Add detection of dead struct fields
Remove dead code identified by the dead field lint