Update extract-tests.py to use same test directives as rustdoc.

Closes #11362.
auto merge of #11490 : wting/rust/wting_11362_update_extract_tests, r=alexcrichton

Refactored the file quite a bit, I can add unit tests if desired. There's a few changes from the previous version's behavior:

- destination directory will be created if it doesn't exist
- strings and file is written as unicode

I have a few questions, but will ask them in #11362.
Implement an unused_result lint

I attempted to implement the lint in two steps. My first attempt was a
default-warn lint about *all* unused results. While this attempt did indeed find
many possible bugs, I felt that the false-positive rate was too high to be
turned on by default for all of Rust.

My second attempt was to make unused-result a default-allow lint, but allow
certain types to opt-in to the notion of "you must use this". For example, the
Result type is now flagged with #[must_use]. This lint about "must use" types is
warn by default (it's different from unused-result).

The unused_must_use lint had a 100% hit rate in the compiler, but there's not
that many places that return Result right now. I believe that this lint is a
crucial step towards moving away from conditions for I/O (because all I/O will
return Result by default). I'm worried that this lint is a little too specific
to Result itself, but I believe that the false positive rate for the
unused_result lint is too high to make it useful when turned on by default.
Flag Result as #[must_use] and deal with fallout.