Include import name in import shadowing error messages.

This partially alleviates the confusing behavior in issue #16597
Update compile-fail tests due to changed diagnostics
Add compile-fail test for missing import shadowing case
libstd: Refactor Duration.

This changes the internal representation of `Duration` from

    days: i32,
    secs: i32,
    nanos: u32

to

    secs: i64,
    nanos: i32

This resolves #16466. Some methods now take `i64` instead of `i32` due
to the increased range. Some methods, like `num_milliseconds`, now
return an `Option<i64>` instead of `i64`, because the range of
`Duration` is now larger than e.g. 2^63 milliseconds.
libstd: Limit Duration range to i64 milliseconds.

This enables `num_milliseconds` to return an `i64` again instead of
`Option<i64>`, because it is guaranteed not to overflow.

The Duration range is now rougly 300e6 years (positive and negative),
whereas it was 300e9 years previously. To put these numbers in
perspective, 300e9 years is about 21 times the age of the universe
(according to Wolfram|Alpha). 300e6 years is about 1/15 of the age of
the earth (according to Wolfram|Alpha).
extern crate foobar as foo;

Implements remaining part of RFC #47.
Addresses issue #16461.

Removed link_attrs from rust.md, they don't appear to be supported by
the parser.

Changed all the tests to use the new extern crate syntax

Change pretty printer to use 'as' syntax
doc: add missing word
doc: use the more convenient 'cargo run' command

This lovely command has already been introduced, so let's take advantage
of it.
doc: remove misleading/confusing info

We have to specify the module and the function name in the example where
the module shares a crate with the executable as well, so remove the
redundant (and potentially confusing) mention.
doc: slight consistency fix

Others in this list are Capitalized, so do it here too.
Replace mention of ~T by Box<T> in the FFI guide.
librustc: Restrict transmute intrinsicck to just rust-intrinsic fn's.
Add tests to make sure intrinsicck doesn't apply to non-intrinsic fn's.
Clarify what tx and rx mean

Add a short explanation of what tx and rx mean in terms of channels.
rustdoc: Don't assume that a doc attribute was sugared: Fixes #15976

As of 8876ce44, `is_sugared_doc` is encoded in metadata, so there is no
need to assume that doc attributes came from sugared comments.
Use cargo run in more places
Always insert methods into the search index, even if we're currently in a private module.

Previously, this caused methods of re-exported types to not be inserted into
the search index. This fix may introduce some false positives, but in my
testing they appear as orphaned methods and end up not being inserted into the
final search index at a later stage.

Fixes issue #11943
libcollections: In tests, remove some uses of deprecated methods and
unused imports.
add missing ! char to feature gate hint
Added a note for usage of abs with ::MIN.