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'
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]
New lint : unused_extern_crate. #10385
Removing unused extern crates.
auto merge of #17134 : vberger/rust/lint_unused_extern_crate, r=alexcrichton

This PR creates a new lint : ``unused_extern_crate``, which do pretty much the same thing as ``unused_import``, but for ``extern crate`` statements. It is related to feature request #10385.

I adapted the code tracking used imports so that it tracks extern crates usage as well. This was mainly trial and error and while I believe all cases are covered, there might be some code I added that is useless (long compile times didn't give me the opportunity to check this in detail).

Also, I removed some unused ``extern crate`` statements from the libs, that where spotted by this new lint.
auto merge of #17155 : steveklabnik/rust/dherman_fixes, r=brson

Fixing more suggestions from @dherman . I made them individual commits in case we want to discuss any of them further.
auto merge of #17162 : sfackler/rust/decorator-traits, r=huonw

The other extension types already worked this way and it can be useful to track some state along with the extension.

I also removed the `BasicMacroExpander` and `BasicIdentMacroExpander` since the span inside of them was never used. The expander function types now directly implement the relevant trait.