New `.await` syntax highlighting.

[rubdos]

Since built-in await syntax landed in nightly a few days ago, it would be nice to have .await highlighted in vim!

[rhysd]

This issue seems already fixed. Because await is a new keyword since Rust2018, simply highlight it as keyword should be ok.

rust.vim/syntax/rust.vim

Line 49 in e99f3f5

syn keyword rustAwait await

[rubdos]

Was indeed fixed in #327.