Add new keywords (#282)

* Add new keywords

* `async` from rust-lang/rfcs#2394;
* `existential` from rust-lang/rfcs#2071.

* Make `existential` a contextual keyword

Thanks @dlrobertson who let me use his PR
#284!

syntax/rust.vim

@@ -21,9 +21,12 @@ syn keyword   rustStructure struct enum nextgroup=rustIdentifier skipwhite skipe
 syn keyword   rustUnion union nextgroup=rustIdentifier skipwhite skipempty contained
 syn match rustUnionContextual /\<union\_s\+\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\)\%([^[:cntrl:][:punct:][:space:]]\|_\)*/ transparent contains=rustUnion
 syn keyword   rustOperator    as
+syn keyword   rustExistential existential nextgroup=rustTypedef skipwhite skipempty contained
+syn match rustExistentialContextual /\<existential\_s\+type/ transparent contains=rustExistential,rustTypedef
 
 syn match     rustAssert      "\<assert\(\w\)*!" contained
 syn match     rustPanic       "\<panic\(\w\)*!" contained
+syn keyword   rustKeyword     async
 syn keyword   rustKeyword     break
 syn keyword   rustKeyword     box nextgroup=rustBoxPlacement skipwhite skipempty
 syn keyword   rustKeyword     continue
@@ -291,6 +294,7 @@ hi def link rustDynKeyword    rustKeyword
 hi def link rustTypedef       Keyword " More precise is Typedef, but it doesn't feel right for Rust
 hi def link rustStructure     Keyword " More precise is Structure
 hi def link rustUnion         rustStructure
+hi def link rustExistential   rustKeyword
 hi def link rustPubScopeDelim Delimiter
 hi def link rustPubScopeCrate rustKeyword
 hi def link rustSuper         rustKeyword