Change method lookup to require invariance for mutable references

Fixes #17033.
Fixes #15965.
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]
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.
Use DWARF4 on Windows.
Fix #17156
Register snapshots
Remove BasicMacroExpander and BasicIdentMacroExpander

The spans inside of these types were always None and never used. Pass
the expander function directly instead of wrapping it in one of these
types.

[breaking-change]
librustc: Make sure region bounds on closures outlive calls to them.

This can break code like:

    fn call_rec(f: |uint| -> uint) -> uint {
        (|x| f(x))(call_rec(f))
    }

Change this code to use a temporary instead of violating the borrow
rules:

    fn call_rec(f: |uint| -> uint) -> uint {
        let tmp = call_rec(|x| f(x)); f(tmp)
    }

Closes #17144.

[breaking-change]
Fix bug in padding unicode, #17105.
librustc: Enum nullable pointer opt should not apply to raw pointers.
Make debug message about resolving `extern crate` statements more helpful
Fix check for existing crate when using --extern

When checking for an existing crate, compare against the
`crate_metadata::name` field, which is the crate name which
was requested during resolution, rather than the result of the
`crate_metadata::name()` method, which is the crate name within
the crate metadata, as these may not match when using the --extern
option to `rustc`.

This fixes spurious "multiple crate version" warnings under the
following scenario:

- The crate `foo`, is referenced multiple times
- `--extern foo=./path/to/libbar.rlib` is specified to rustc
- The internal crate name of `libbar.rlib` is not `foo`

The behavior surrounding `Context::should_match_name` and the
comments in `loader.rs` both lead me to believe that this scenario
is intended to work.

Fixes #17186
The example code uses trigger_callback(), not do_callback().
Use a space after colons per the Rust coding style:
https://github.com/rust-lang/rust-guidelines/blob/master/style/whitespace.md