Change method lookup to require invariance for mutable references

Fixes #17033.
Fixes #15965.
fixes for Box<[T]>

The pointer in the slice must not be null, because enum representations
make that assumption. The `exchange_malloc` function returns a non-null
sentinel for the zero size case, and it must not be passed to the
`exchange_free` lang item.

Since the length is always equal to the true capacity, a branch on the
length is enough for most types. Slices of zero size types are
statically special cased to never attempt deallocation. This is the same
implementation as `Vec<T>`.

Closes #14395
Add links to forums in README.md, including discuss.rust-lang.org
Fix naming of windows installer
Implement tuple and tuple struct indexing

This allows code to access the fields of tuples and tuple structs:

    let x = (1i, 2i);
    assert_eq!(x.1, 2);

    struct Point(int, int);
    let origin = Point(0, 0);
    assert_eq!(origin.0, 0);
    assert_eq!(origin.1, 0);
Avoid ever constructing cyclic types in the first place, rather than detecting them in resolve. This simplifies logic elsewhere in the compiler. cc #5527
Prefer lualatex, xelatex, pdflatex, in that order. #12804
Don't ICE on macros with -Z show-span

Closes #17115
Change ItemModifier and ItemDecorator to traits

For convenience, the traits are implemented for the respective bare
functions. Change code from this:

```rust
ItemDecorator(some_function)
// or
ItemModifier(some_other_function)
```
to
```rust
ItemDecorator(box some_function)
// or
ItemModifier(box some_other_function)
```

[breaking-change]
auto merge of #16680 : brson/rust/readme, r=steveklabnik

This adds links to SO, reddit, and discuss to the README. The main intent is to start advertising discuss.rust-lang.org more, in a location that doesn't mislead casual users to it (people who are building Rust are more likely to be the right audience than those that are just visiting the web site).
auto merge of #17135 : brson/rust/wininst, r=alexcrichton

This builds on https://github.com/rust-lang/rust/pull/17109, putting the target triple into the installer name so that we can have both 32-bit and 64-bit.

The resulting installers will be called `rust-0.12.0-pre-x86_64-w64-mingw32.exe`, etc.
reintroduce manual link in doc index

whoops

Fixes https://github.com/rust-lang/rust/pull/16827#issuecomment-55160273
Fix vector terminology in the manual.

fixes #17148
dave hates jokes :wink:
hello_world.rs -> main.rs
Remove much of the modules section.

This part can get _really_ confusing, and we want to make sure that
people succeed in the guide. I plan on making a module guide in the
future to replace the information here.
remove rich hickey love
don't say 'semantic'
Disable ASLR on Windows, for now.
Escape backslashes in debugger scripts.