Auto merge of rust-lang#3330 - RossSmyth:win-fmt, r=RalfJung

Fix .\miri fmt on Windows

This allows .\miri fmt to work on Windows. Closes rust-lang#3317.

To reiterate, the problem with using `miri fmt` on Windows is that the CLI arguments to rustfmt are too long. Currently over 65,000 characters are used in the call to rustfmt, [which is incompatible with Windows](https://devblogs.microsoft.com/oldnewthing/20031210-00/?p=41553) as it is limited to (2^15 - 1) for all arguments plus all env vars.

Two things are done do get around this limit:

1. Call out to cargo-fmt for the crates that exist.
2. Batch rustfmt calls by length

Another alternative would be to just use rustfmt for everything and don't use cargo-fmt for the crates.

I don't know how much you guys may care about `miri fmt` time to run. I don't know the diff as it did not work before on my computer.

[I have another branch that solves this, but in a less permanent way](RossSmyth/miri/tree/windows-fmt). That was my initial attempt, and I learned that even with cargo-fmt and relative paths, the rustfmt call still has 27k characters. This is closer to the limit than I expected, so it would not be a permanent solution. So I went back to absolute paths & batching.

Author: bors