Fix some issues with directive definitions

I found some issues with directive definitions:

- I couldn't use `on FIELD_DEFINITION`, I believe because `FIELD` was parsed
  first in `executableDirectiveLocation`. I've combined both
  `executableDirectiveLocation` and `typetypeSystemDirectiveLocation` into one
  function which can reorder them to ensure every directive location gets a fair
  chance at parsing.

Not actually to do with directives, some literals weren't being parsed
correctly.

- The GraphQL spec defines list to be `[]` or `[Value]`, but empty literal lists
  weren't being parsed correctly because of using `some` instead of `many`.

- The GraphQL spec defines objects to be `{}` or `{Name: Value}`, but empty
  literal objects had the same issue.

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to
 - `Language.GraphQL.Execute.OrderedMap` is a map data structure, that preserves
   insertion order.
 
+### Fixed
+- Parser now accepts empty lists and objects.
+- Parser now accepts all directive locations.
+
 ## [0.11.1.0] - 2021-02-07
 ### Added
 - `Validate.Rules`:
@@ -109,7 +113,7 @@ and this project adheres to
   `locations`.
 - Parsing comments in the front of definitions.
 - Some missing labels were added to the parsers, some labels were fixed to
-  refer to the AST nodes being parsed. 
+  refer to the AST nodes being parsed.
 
 ### Added
 - `AST` reexports `AST.Parser`.