Add .await syntax highlighting (#327)

syntax/rust.vim

@@ -26,7 +26,7 @@ syn match rustExistentialContextual /\<existential\_s\+type/ transparent contain
 
 syn match     rustAssert      "\<assert\(\w\)*!" contained
 syn match     rustPanic       "\<panic\(\w\)*!" contained
-syn match     rustKeyword     "\<async\%(\s\|\n\)\@="
+syn match     rustAsync       "\<async\%(\s\|\n\)\@="
 syn keyword   rustKeyword     break
 syn keyword   rustKeyword     box nextgroup=rustBoxPlacement skipwhite skipempty
 syn keyword   rustKeyword     continue
@@ -45,7 +45,8 @@ syn keyword   rustKeyword     use nextgroup=rustModPath skipwhite skipempty
 " FIXME: Scoped impl's name is also fallen in this category
 syn keyword   rustKeyword     mod trait nextgroup=rustIdentifier skipwhite skipempty
 syn keyword   rustStorage     move mut ref static const
-syn match rustDefault /\<default\ze\_s\+\(impl\|fn\|type\|const\)\>/
+syn match     rustDefault     /\<default\ze\_s\+\(impl\|fn\|type\|const\)\>/
+syn keyword   rustAwait       await
 
 syn keyword rustPubScopeCrate crate contained
 syn match rustPubScopeDelim /[()]/ contained
@@ -344,6 +345,8 @@ hi def link rustExternCrate   rustKeyword
 hi def link rustObsoleteExternMod Error
 hi def link rustBoxPlacementParens Delimiter
 hi def link rustQuestionMark  Special
+hi def link rustAsync         rustKeyword
+hi def link rustAwait         rustKeyword
 
 " Other Suggestions:
 " hi rustAttribute ctermfg=cyan