Add example for str replace() and MaybeOwned

  - for 3 implementations of into_maybe_owned()
  - is_slice()
  - is_owned()
Add an iterate function to core::iter

Implementation by Kevin Ballard.

The function returns an Unfold iterator producing an infinite stream
of results of repeated applications of the function, starting from
the provided seed value.
auto merge of #15507 : jakub-/rust/iterate, r=alexcrichton

The new iterator takes a function and produces an infinite stream
of results of repeated applications of the function, starting from
the provided seed value.
refactor Method definition to make space for macros

This change propagates to many locations, but because of the
Macro Exterminator (or, more properly, the invariant that it
protects), macro invocations can't occur downstream of expansion.
This means that in librustc and librustdoc, extracting the
desired field can simply assume that it can't be a macro
invocation. Functions in ast_util abstract over this check.
test case for method macros
macro method unit test case fix
update fold_method to return a smallvector

This is nice for macros, to allow them to expand into multiple methods
remove no-stmt check

nothing wrong with a statement expanding into 0 stmts, that I can see.
macro in method position parsing
add make_method method to MacResult trait

this allows macro results to be parsed as methods
expansion abstraction
macro expansion for methods

Closes #4621
auto merge of #15646 : jbclements/rust/method-macros, r=cmr

This patch adds support for macros in method position. It follows roughly the template for Item macros, where an outer `Method` wrapper contains a `Method_` enum which can either be a macro invocation or a standard macro definition. 

One note; adding support for macros that expand into multiple methods is not included here, but should be a simple parser change, since this patch updates the type of fold_macro to return a smallvector of methods.

For reviewers, please pay special attention to the parser changes; these are the ones I'm most concerned about.

Because of the small change to the interface of fold_method, this is a ...

[breaking change]
libstd: Add a few doc examples

This patch adds doc examples for the make_absolute, change_dir,
errors_string and args functions in the os module.
Stabilization for `owned` (now `boxed`) and `cell`

This PR is the outcome of the library stabilization meeting for the
`liballoc::owned` and `libcore::cell` modules.

Aside from the stability attributes, there are a few breaking changes:

* The `owned` modules is now named `boxed`, to better represent its
  contents. (`box` was unavailable, since it's a keyword.) This will
  help avoid the misconception that `Box` plays a special role wrt
  ownership.

* The `AnyOwnExt` extension trait is renamed to `BoxAny`, and its `move`
  method is renamed to `downcast`, in both cases to improve clarity.

* The recently-added `AnySendOwnExt` extension trait is removed; it was
  not being used and is unnecessary.

[breaking-change]
auto merge of #15591 : aturon/rust/box-cell-stability, r=alexcrichton

This PR is the outcome of the library stabilization meeting for the
`liballoc::owned` and `libcore::cell` modules.

Aside from the stability attributes, there are a few breaking changes:

* The `owned` modules is now named `boxed`, to better represent its
  contents. (`box` was unavailable, since it's a keyword.) This will
  help avoid the misconception that `Box` plays a special role wrt
  ownership.

* The `AnyOwnExt` extension trait is renamed to `BoxAny`, and its `move`
  method is renamed to `downcast`, in both cases to improve clarity.

* The recently-added `AnySendOwnExt` extension trait is removed; it was
  not being used and is unnecessary.

[breaking-change]
native: Search the child's PATH on win32

In order to have the spawning semantics be the same for unix/windows, the
child's PATH environment variable needs to be searched rather than the parent's
environment variable.

If the child is inheriting the parent's PATH, then no action need be taken as
windows will do the heavy lifting. If the child specifies its own PATH, then it
is searched beforehand for the target program and the result is favored if a hit
is found.

cc #15149, but does not close the issue because libgreen still needs to be
updated.
auto merge of #15158 : alexcrichton/rust/windows-paths, r=brson

In order to have the spawning semantics be the same for unix/windows, the
child's PATH environment variable needs to be searched rather than the parent's
environment variable.

If the child is inheriting the parent's PATH, then no action need be taken as
windows will do the heavy lifting. If the child specifies its own PATH, then it
is searched beforehand for the target program and the result is favored if a hit
is found.

cc #15149, but does not close the issue because libgreen still needs to be
updated.
std: make std::io::IoError{,Kind} implement Eq
auto merge of #15649 : catharsis/rust/rust-libstd-examples, r=alexcrichton

This patch adds doc examples for the make_absolute, change_dir,
errors_string and args functions in the os module.