grammar,compiler: Add Support For Fixed-Width Integers

Distinguishing between signed and unsigned fixed-width integer
types must still be done.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

tree-sitter-p4/test/corpus/statements.txt

@@ -38,7 +38,9 @@ int fun() {
   (function_declaration
    (typeRef
     (baseType
-     (int)
+     (int_type
+      (int)
+     )
     )
    )
    (identifier)
@@ -382,7 +384,9 @@ parser simple() {
        (variableDeclaration
         (typeRef
          (baseType
-          (int)
+          (int_type
+           (int)
+          )
          )
         )
         (identifier)