Make rustfmt#Format work with Vim <7.2.248.
Make rustfmt#Format work with Vim <7.2.248.

`systemlist()` was only introduced in Vim 7.2.248. If it’s unavailable
we can just use `split(system(…), '\r\?\n')` instead. (We still use
`systemlist()` if it’s available under the assumption it’ll be faster.)

Thanks to @timothy-mcroy for the original bug report.

Closes #66.

rust-lang/rust.vim