Rollup merge of #60971 - rbtcollins:docs-perf, r=rbtcollins,GuillaumeGomez

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 both consistently reduce syscalls as well as make
  deferring to threads cleaner.
* 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.
* 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
user    0m0.015s
sys     0m0.000s

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
user    0m0.000s
sys     0m0.015s

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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