Add new keywords

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

Author: hcpl

syntax/rust.vim

@@ -24,10 +24,12 @@ syn keyword   rustOperator    as
 
 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
 syn keyword   rustKeyword     crate
+syn keyword   rustKeyword     existential
 syn keyword   rustKeyword     extern nextgroup=rustExternCrate,rustObsoleteExternMod skipwhite skipempty
 syn keyword   rustKeyword     fn nextgroup=rustFuncName skipwhite skipempty
 syn keyword   rustKeyword     in impl let