Fix grammar for empty list literals (#645)

The current grammar forbids: `[] : List a` because it uses `import-expression`
for the type annotation, which in turn forbids an unparenthesized
application of the `List` type constructor to `a`.  Changing it to
`application-expression` fixes that (which makes sense, since `List a`
is an application expression).

standard/dhall.abnf

@@ -706,7 +706,7 @@ expression =
     ; NOTE: Backtrack if parsing this alternative fails since we can't tell
     ; from the opening bracket whether or not this will be an empty list or
     ; a non-empty list
-    / "[" whsp "]" whsp ":" whsp1 import-expression
+    / "[" whsp "]" whsp ":" whsp1 application-expression
 
     ; "toMap e : t"
     ;