Add signed keyword

Fixes #10

corpus/types.txt

@@ -46,7 +46,7 @@ Type modifiers
 
 void f(unsigned);
 void f(unsigned int);
-void f(unsigned long int);
+void f(signed long int);
 void f(unsigned v1);
 void f(unsigned long v2);
 

grammar.js

@@ -282,6 +282,7 @@ module.exports = grammar({
 
     sized_type_specifier: $ => seq(
       repeat1(choice(
+        'signed',
         'unsigned',
         'long',
         'short'

src/grammar.json

@@ -1891,6 +1891,10 @@
           "content": {
             "type": "CHOICE",
             "members": [
+              {
+                "type": "STRING",
+                "value": "signed"
+              },
               {
                 "type": "STRING",
                 "value": "unsigned"