std: Remove generics from Option::expect

This commit removes the <M: Any + Send> type parameter from Option::expect in
favor of just taking a hard-coded `&str` argument. This allows this function to
move into libcore.

Previous code using strings with `expect` will continue to work, but code using
this implicitly to transmit task failure will need to unwrap manually with a
`match` statement.

[breaking-change]
Closes #14008
auto merge of #14632 : luqmana/rust/cu, r=huonw

The distinction doesn't make sense any more since we don't have do blocks anymore.
rustdoc: Put primitives in respective modules

The logical location for the documentation of a primitive is in the module that
declared it was a module for that primitive.
auto merge of #14628 : luqmana/rust/fcr, r=nikomatsakis

#14589.
Fixes #13843.

An empty regex is a valid regex that always matches. This behavior
is consistent with at least Go and Python.

A couple regression tests are included.
Some minor documentation touchups for libregex. Fixes #13800.
syntax: shuffle some allocation out of binop_to_str
Add comments for the token table
auto merge of #14634 : BurntSushi/rust/fix-13843, r=alexcrichton

An empty regex is a valid regex that always matches. This behavior
is consistent with at least Go and Python.

A couple regression tests are included.

I'd just assume that an empty regex is an invalid regex and that an error should be returned (I can't think of any reason to use an empty regex?), but it's probably better to be consistent with other regex libraries.
auto merge of #14635 : BurntSushi/rust/regex-doco-touchups, r=alexcrichton
libc: only provide an rlib.

There's absolutely no reason for `libc` to be offered as a dynamic
library.
Add code example to std::os::getenv for unix.
Implement Show for RingBuf
auto merge of #14616 : forticulous/rust/rc-show, r=alexcrichton

Show impl for Rc
Mark the exit of infinite loops as unreachable
Fix an ICE when a function argument is of the bottom type

Fixes #13352
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.
auto merge of #14623 : exscape/rust-fork/master, r=alexcrichton

Unlike ImmutableClonableVector::permutations() which returns an iterator,
cloning the entire array each iteration, these methods mutate the vector in-place.
For that reason, these methods are much faster; between 35-55 times faster,
depending on the benchmark. They also generate permutations in lexicographical order.
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.
syntax: methodify the lexer