rust.md: Explicitly point out how special `'static` is.

Drive-by: fix description of `&content` to point out that `&'f type`
is (as of today) only for type expressions.
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Fix many-crates-but-no-match test. (Issue #16348)
librustc: Use separate stack slot for each return.
Reenable ignored test and add run-pass test.
librustc: Add an intrinsic to retrieve the return pointer of a function.

This is needed for some GC stuff in Servo.
librustc: Don't allow return_address intrinsic in functions that don't use an out pointer.
librustc: Don't use Load/Store for structural values.
Change std::fmt::{Float,LowerExp,UpperExp} to not print '-NaN' for f32::NAN and f64::NAN
librustc: Don't use an alloca per return if the function doesn't have nested returns.
guide-runtime.md: remove redundant verb
guide-testing.md: add auxiliary verb
guide-unsafe.md: fix noun
Replace all references to "Win32" with "Windows".

For historical reasons, "Win32" has been used in Rust codebase to mean "Windows OS in general".
This is confusing, especially now, that Rust supports Win64 builds.

[breaking-change]
Replace #[cfg(target_os = "win32")] with #[cfg(target_os = "windows")]
Minor changes to `rust.md`, and `guide-ffi.md`.

  * `rust.md`: whanges for consistency

  * `guide-ffi.md`: wrapped inline code
Replace "ignore-win32" in tests with "ignore-windows"
auto merge of #16425 : nham/rust/fix_nan_format, r=alexcrichton

Currently, this:

    println!("{}", std::f64::NAN);

prints "-NaN". This commit is an attempt to change that to "NaN" instead.
auto merge of #16434 : vadimcn/rust/many-crates-but-no-match, r=alexcrichton
~ -> Box in the manual

Fixes #16439