Fix lexing of await in Rust lexer (#1587)

Currently, `await` is tokenised as `Name::Property`. It should be tokenised as
`Keyword` in the same way as `async` is tokenised. This commit fixes that
mistake.

lib/rouge/lexers/rust.rb

@@ -99,6 +99,7 @@ def macro_closed?
         rule %r/[*\/!@~&+%^<>=\?-]|\.{2,3}/, Operator
 
         rule %r/([.]\s*)?#{id}(?=\s*[(])/m, Name::Function
+        rule %r/[.]\s*await\b/, Keyword
         rule %r/[.]\s*#{id}/, Name::Property
         rule %r/[.]\s*\d+/, Name::Attribute
         rule %r/(#{id})(::)/m do