Fill out the remaining functionality in io::file

This adds bindings to the remaining functions provided by libuv, all of which
are useful operations on files which need to get exposed somehow.

Some highlights:

* Dropped `FileReader` and `FileWriter` and `FileStream` for one `File` type
* Moved all file-related methods to be static methods under `File`
* All directory related methods are still top-level functions
* Created `io::FilePermission` types (backed by u32) that are what you'd expect
* Created `io::FileType` and refactored `FileStat` to use FileType and
  FilePermission
* Removed the expanding matrix of `FileMode` operations. The mode of reading a
  file will not have the O_CREAT flag, but a write mode will always have the
  O_CREAT flag.

Closes #10130
Closes #10131
Closes #10121