Async.Promise: Add l: prefix to variable for funcref

Without l: prefix, "E705: Variable name conflicts with existing function" will be thrown
if user defines same name global function.

Author: lambdalisue

autoload/vital/__vital__/Async/Promise.vim

@@ -278,7 +278,7 @@ function! s:_chain(promise_factories, results) abort
   if len(a:promise_factories) is# 0
     return s:resolve(a:results)
   endif
-  let Factory = remove(a:promise_factories, 0)
+  let l:Factory = remove(a:promise_factories, 0)
   try
     return Factory()
           \.then({ v -> add(a:results, v) })