guide-runtime.md: remove redundant verb
guide-testing.md: add auxiliary verb
guide-unsafe.md: fix noun
guide-unsafe.md: fix wording

Following a suggestion by Huon Wilson.
quote_expr macro: embed Ident using special encoding that preserves hygiene.

This adds support to `quote_expr!` and friends for round-trip hygienic
preservation of Ident.

Here are the pieces of the puzzle:

* adding a method for encoding Ident for re-reading into token tree.

* Support for reading such encoded Idents in the lexer.  Note that one
  must peek ahead for MOD_SEP after scan_embedded_hygienic_ident.

* To ensure that encoded Idents are only read when we are in the midst
  of expanding a `quote_expr` or similar, added a
  `read_embedded_ident` flag on `StringReader`.

* pprust support for encoding Ident's as (uint,uint) pairs (for hygiene).
rustc lexer: regression tests for embedded Idents.

I chose to make two of them because I wanted something close to an
"end-to-end" test (*), but at the same time I wanted a test that
would run on Windows (**).

(*) The run-make test serves as the end-to-end: It constructs an input
that is trying to subvert the hack and we are going to check that it
fails in the attempt).

(**) The compile-fail-fulldeps test serves as a more narrow test that
will be tested on all platforms.  It also attempts to subvert the
hack, testing that when you use `new_parser_from_tts`, the resulting
parser does not support reading embedded Idents.
std: Add a Duration type

Taken from rust-chrono[1]. Needed for timers per #11189.
Experimental.

[1]: https://github.com/lifthrasiir/rust-chrono
std: Rename sleep, periodic, and oneshot timers to sleep_ms, etc.

Rename io::timer::sleep, Timer::sleep, Timer::oneshot,
Timer::periodic, to sleep_ms, oneshot_ms, periodic_ms. These functions
all take an integer and interpret it as milliseconds.

Replacement functions will be added that take Duration.

[breaking-change]
std: Add sleep, oneshot and periodic timers, taking Duration
std: Add comments to the time module
std: Update Duration from upstream

From rust-chrono 4f34003e03e259bd5cbda0cb4d35325861307cc6
std: Change time::MAX to time::MAX_DURATION, etc.
std: Remove ms-taking methods from timers
std: Remove the `zero` constructor from `Duration`

This is a workaround for having to write `Zero::zero` and will
be solved at the language level someday.
std: Restore missing timer examples
std: Clarify what timers do with zero and negative durations

Add tests. Also fix a bunch of broken time tests.
std: Make the TCP/UDP connect_timeout methods take Duration

[breaking-change]
std: connect_timeout requires a positive Duration

This is only breaking if you were previously specifying a duration
of zero for some mysterious reason.

[breaking-change]
std: Refactor time module a bit

Put `Duration` in `time::duration`, where the two constants can
be called just `MAX` and `MIN`. Reexport from `time`.
This provides more room for the time module to expand.
Fix compiletest to use Duration