Use defvar for rust-top-item-beg-re instead

Since rust-top-item-beg-re hasn't been defined yet, using defvar instead of
setq is more appropriate here (and also silences compilation warnings).
Implement fallbacks for functions unavailable in older versions of Windows
etc: Add box as a keyword to editor configs

This keyword is about to be used much more often than it was a few weeks ago,
it's time to tell editors about the new keyword.
Refactor writeback code. cc #5527
Remove error message that refactor suppresses
Revise doc-comments for graphviz to avoid generating files from rustdoc runs.

Fix #13965.

There is a dance here between the `main` that actually runs versus the
`main` that is printed in the output documentation.  We don't run the
latter `main`, but we do at least compile (and thus type-check) it.
It is still the responsibility of the documenter to ensure that the
signatures of `fn render` are kept in sync across the blocks.
rustc: Fix enum variant privacy across crates

The code in resolve erroneously assumed that private enums weren't visited, so
the logic was adjusted to check to see if the enum definition itself was public.

Closes #11680
Add documentation for Windows compatibility layer and remove unneeded attributes
Add testcase for #11493
Move Windows compatibility layer to libnative
std: Remove bump_box_refcount. Deprecated and unused. Deprecused.
std: Doc typos
std: Reorder definitions in cast

Prioritize `transmute` and `forget`.
std: Change names of transmute's type parameters

from L, G to T, U. I don't know what L and G mean.
T, U easier to understand.
std: Small doc tweaks
Change the biguint documentation example to reflect deprecated owned vector
auto merge of #13976 : pnkfelix/rust/fsk-fix-13965, r=alexcrichton

Fix #13965.

This commit adopts the second strategy I outlined in #13965, where the bulk of the code is still "smoke tested" (in the sense that rustdoc attempts to run it, sending all of the generated output into a locally allocated `MemWriter`).  The part of the code that is ignored (but included in the presentation) is isolated to a three-line `main` function that invokes the core rendering routine.

In the generated rustdoc output, this leads to a small break between the two code blocks, but I do not think this is a large issue.
mk: Fix make install

Forgot to update the installation procedure with the knowledge that libcore is
only available as an rlib, not as a dylib.

Closes #14026
std: Add I/O timeouts to networking objects

These timeouts all follow the same pattern as established by the timeouts on
acceptors. There are three methods: set_timeout, set_read_timeout, and
set_write_timeout. Each of these sets a point in the future after which
operations will time out.

Timeouts with cloned objects are a little trickier. Each object is viewed as
having its own timeout, unaffected by other objects' timeouts. Additionally,
timeouts do not propagate when a stream is cloned or when a cloned stream has
its timeouts modified.

This commit is just the public interface which will be exposed for timeouts, the
implementation will come in later commits.
native: Implement timeouts for unix networking

This commit has an implementation of the previous commit's timeout interface for
I/O objects on unix platforms. For implementation details, see the large comment
at the end of libnative/io/net.rs which talks about the general strategy taken.

Thankfully, all of these implementations can share code because they're
performing all the same operations.

This commit does not implement timeouts for named pipes on windows, only tcp/udp
objects on windows (which are quite similar to their unix equivalents).