librustc: Fix field offsets for packed structs.
Add test for passing/getting packed structs with ffi.
auto merge of #16575 : pcwalton/rust/import-foo-as-bar, r=aturon

of `use bar as foo`.

Change all uses of `use foo = bar` to `use bar as foo`.

Implements RFC #47.

Closes #16461.

[breaking-change]

r? @aturon
Explain EOF behavior in File.eof().

Fies #16239.
work around MinGW linker bug for ASLR on Windows

Closes #16514
Explain modulo in the guide.

Fixes #15954
auto merge of #16578 : steveklabnik/rust/gh16239, r=pcwalton

Fies #16239.
Fixing bitvset is_disjoint, fixes #16587
Make comment about small bias in %.

Fixes #16354.
Fix underflow bug in core::str::Searcher::new for haystacks of length < 20
auto merge of #16579 : steveklabnik/rust/gh9099, r=pcwalton

Fixes #9099
auto merge of #16580 : steveklabnik/rust/gh1498, r=pcwalton

Fixes #14948
auto merge of #16345 : EduardoBautista/rust/fix-error-message-in-guide, r=steveklabnik

Just some simple changes to the guide.
Add a test for the fix of issue 16589
auto merge of #16573 : ruud-v-a/rust/timespec-arithmetic, r=alexcrichton

This implements `Add` and `Sub` for `Timespec`, which enables `Timespec` to be used as a time span. For example:

```rust
let begin = get_time();
// Do some stuff.
let end = get_time();
let delta = end - begin;
println!("Doing stuff took {}.{:09d} seconds.", delta.sec, delta.nsec);
```
This resolves one of the points mentioned in #2153.
A few minor documentation fixes
auto merge of #16364 : tbu-/rust/pr_checkeddiv0, r=alexcrichton
Include import name in import shadowing error messages.

This partially alleviates the confusing behavior in issue #16597
auto merge of #16582 : Gankro/rust/bitv, r=alexcrichton

This was bothering me (and some other people). The macro was necessary in a transient step of my development, but I converged on a design where it was unnecessary, but it didn't really click that that had happened.

This fixes it up.
auto merge of #16583 : steveklabnik/rust/gh16569, r=pcwalton

Fixes #16569