readme: Link 'Using' wiki page instead of 'Getting started'.

The Windows-specific instruction under 'Quick Start' linked the wiki article on getting started developing Rust itself, but the quick start is just about obtaining a working Rust installation. The actual wiki page with Windows-specific instructions was difficult to find.
In sieve example, end iteration sooner

The Sieve algorithm only requires checking all elements up to and including the square root of the maximum prime you're looking for. After that, the remaining elements are guaranteed to be prime.
Update README.md

Added 64-bit Windows support to the list of platforms. Support for 64-bit Windows was added in Rust 0.12.0. See  https://mail.mozilla.org/pipermail/rust-dev/2014-October/011267.html for the release notes.
Teach libsyntax about `while let`
Desugar `while let` into `loop { match { ... } }`
Move `while let` behind `while_let` feature gate
Handle `while let` desugaring
Add tests for `while let`
Add `while let` to the reference

Closes #17687
Use explicit case rather than catch-all
Fix HTML version of the keyword table.

This should really fix rust-lang/rust#17528.
src/doc/README.md: fix a dead link.

Also make some typographical changes.
Fix run-pass/lang-item-public test on FreeBSD
fix broken link in guide-strings.md
rustc: Warn about dead constants

A few catch-all blocks ended up not having this case for constants.

Closes #17925
Guide: remove promise of a counter in the guessing game.

Issue #17964.
Fix `find` check for executables

Apparently the fix in d08441b9 didn't catch a recent executable (#17965), but
I've verified that this does indeed catch the executable
Properly visit nested functions and closures in check_rvalues

This correctly catches moves of unsized values in nested
functions and closures.

Closes issue #17651
Fixes small error on the doc (task part)
Add regression test for issue #17651