This commit deprecates the majority of std::old_io::fs in favor of std::fs and
its new functionality. Some functions remain non-deprecated but are now behind a
feature gate called `old_fs`. These functions will be deprecated once
suitable replacements have been implemented.
The compiler has been migrated to new `std::fs` and `std::path` APIs where
appropriate as part of this change.
This pulls all of our long-form documentation into a single document,
nicknamed "the book" and formally titled "The Rust Programming
Language."
A few things motivated this change:
* People knew of The Guide, but not the individual Guides. This merges
them together, helping discoverability.
* You can get all of Rust's longform documentation in one place, which
is nice.
* We now have rustbook in-tree, which can generate this kind of
documentation. While its style is basic, the general idea is much
better: a table of contents on the left-hand side.
* Rather than a almost 10,000-line guide.md, there are now smaller files
per section.
