Add DocFS layer to rustdoc

* Move fs::create_dir_all calls into DocFS to provide a clean
  extension point if async extension there is needed.
* Convert callsites of create_dir_all to ensure_dir to reduce syscalls.
* Convert fs::write usage to DocFS.write
  (which also removes a lot of try_err! usage for easier reading)
* Convert File::create calls to use Vec buffers and then DocFS.write
  in order to consistently reduce syscalls as well, make
  deferring to threads cleaner and avoid leaving dangling content if
  writing to existing files....
* Convert OpenOptions usage similarly - I could find no discussion on
  the use of create_new for that one output file vs all the other
  files render creates, if link redirection attacks are a concern
  DocFS will provide a good central point to introduce systematic
  create_new usage. (fs::write/File::create is vulnerable to link
  redirection attacks).
* DocFS::write defers to rayon for IO on Windows producing a modest
  speedup: before this patch on my development workstation:

$ time cargo +mystg1 doc -p winapi:0.3.7
 Documenting winapi v0.3.7
    Finished dev [unoptimized + debuginfo] target(s) in 6m 11s

real    6m11.734s

Afterwards:
$ time cargo +mystg1 doc -p winapi:0.3.7
   Compiling winapi v0.3.7
 Documenting winapi v0.3.7
    Finished dev [unoptimized + debuginfo] target(s) in 49.53s

real    0m49.643s

I haven't measured how much time is in the compilation logic vs in the
IO and outputting etc, but this takes it from frustating to tolerable
for me, at least for now.

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