add rustfmt support

[Dangerise]

implement #1901

closes #1906

[ealmloff]

It looks like cargo fmt errors are getting captured somewhere without warning the user.

With this code:

fn main() {

Running dx fmt --rustfmt gives no output and does not format the code
Running cargo fmt gives this warning:

 --> /../testing/src/main.rs:1:12
  |
1 | fn main() {
  |           -^
  |           |
  |           unclosed delimiter

[Dangerise]

@ealmloff

I've got some problems about dx fmt

In the gif above , dx was built from crates.io, dx-git was built from github

also, I try to print out the files that dx fmt found, I found it empty.

[Dangerise]

• replace gitignore with ignore crate
• rewrite autoformat_project with rayon
• use prettyplease as formatter

[Dangerise]

fixed #1906

[Dangerise]

@ealmloff updated

[ealmloff]

I pushed a few changes to make error reporting better and rename the rustfmt flag, other than that this looks good, thank you!