refactor(api): use typed keysets

Initially this is just for geting rid of boilerplate,
but eventually the types could get exposed as metadata

src/nvim/api/private/defs.h

@@ -124,10 +124,20 @@ struct key_value_pair {
   Object value;
 };
 
-typedef Object *(*field_hash)(void *retval, const char *str, size_t len);
+typedef uint64_t OptionalKeys;
+
+// this is the prefix of all keysets with optional keys
+typedef struct {
+  OptionalKeys is_set_;
+} OptKeySet;
+
 typedef struct {
   char *str;
   size_t ptr_off;
+  ObjectType type;  // kObjectTypeNil == untyped
+  int opt_index;
 } KeySetLink;
 
+typedef KeySetLink *(*FieldHashfn)(const char *str, size_t len);
+
 #endif  // NVIM_API_PRIVATE_DEFS_H